Both Balraj Sahni and his wife Damyanti joined BBC London as Hindi announcers. His stint in London drew him to one of the most significant influences of his life: Marxism. World War II was brewing and he had a ringside view. The horrors and machinations of war in this specific juncture of history would have given him the intellectual impetus to start seeing the world with Marxist lens. What accentuated this was his exposure to Soviet Cinema. He was disillusioned with the lack of realism in American cinema of the time. Driven by Communist ideals, the Soviet films he saw portrayed class struggles and the daily battles foughts by common men and women. These films became a source of inspiration and his introduction to Marxism and Leninism, which, he concluded, offered the solution to most of mankind's problems. During this period he was also introduced to the concept of “People's Theatre”, plays performed by the working class about their concerns and struggles. This exposure solidifed Balraj's views about art and artists. The egalitarian ideals already sparked by his work with Rabindranath Tagore and Mahatma Gandhi was taken to the next level and firmly shaped his worldview, which dominated his life and work for all time to come.

He returned to India and nursed the idea of working in films. He bumped into Chetan who offered him work in his latest film, Neecha Nagar. Though the role didn't materialise, Sahni was drawn to the world of films. HIs socialistic inclimatiotions inducted him into Indian People's Theatre Association (IPTA), of which he soon became a pillar. He was given to direct Khwaja Ahmad Abbas's first play Zubeida, and acted in Mohan Segal's play Jadu ki Kursi. His most significant collaboration during the IPTA years was being cast in Khwaja Ahmad Abba's directorial debut, the seminal Dharti Ke Lal. It was IPTA's first and only official film venture, and Balraj Sahni's screen debut. The film and its realistic depiction of farmers during the famine was pathbreaking for its time. It was also the first Indian film which was dubbed and released in Soviet Russia. Balraj Sahni played the role of Niranjan, an upright and consciential farmer, and Damyanti played his wife Binodini. Although the role didn't earn him any acting laurels, it was a significant film and he was deeply involved with the project, which gave him the opportunity to participate in the film equivalent of “people's theatre”. He cherished the experience. But the phenomenon didn't catch on, and IPTA was unable to make another film. The same year, filmmaker Phani Majumdar cast Balraj and Damyanti Sahni in his film Door Chalen.

Balraj and Damyanti were paired once again in Gudia, a film adaptation of Henrik Ibsen's A Doll's House, directed by Achyutrao Ranade. Soon after, Damyanti Sahni, Balraj's wife of ten years, passed away at the age of 26. She had contracted a form of amoebic dysentery during the shoot of Dharti Ke Lal, possibly from drinking contaminated water. Overcome with grief, Balraj left Bombay with his children in tow, heading to Rawalpindi. India had just attained independence and with it came partition. The nation was beset with communal riots. Punjab was burning, and Balraj had to move again, this time to the relatively placid Srinagar. Eminent Hindi writer Amritlal Nagar's offer to act in his film Gunjan brought him back to Bombay.

In March 1949, Balraj Sahni married his cousin Santosh. The communist party took a radical stand against the government and it had an impact on IPTA members. In early 1949, while rehearsing for Balwant Gargi's play Signalman Dulee, Balraj was called upon to join a party procession that resulted in him being arrested. Balraj was in prison for six months. During the intervening period, he was required to shoot for Hulchul, directed by S.K. Ojha and produced by K. Asif. With Asif's intervention, Balraj used to be let out briefly to shoot for the part.
Ironically, he was playing a jailor in the film, and his prison exposure served to prepare him for the role. Hulchul was his first major film, in which his costars were Dilip Kumar and Nargis. By the time Balraj was released from prison, his relationship with IPTA had soured, and his finances were dwindling. He and his wife santosh worked on the Hindi dubbing of a Russian film to get by. In the same building, his old friend Chetan Anand had rented an office for his family production house Navketan. Chetan invited him to write the screenplay of his next film Baazi, to be directed by a youngster called Guru Dutt.

Guru Dutt and Balraj Sahni had differing approaches to cinema, which led to several disagreements during the writing of Baazi. For Sahni, the structural integrity of a screenplay was paramount, while Guru Dutt placed greater emphasis on song sequences. Although Sahni had already included six song situations in his script, Guru Dutt added a few more, which drove a wedge into their working relationship.

During a bus commute, Sahni came across a talented conductor named Badruddin Qazi. It was Sahni who advised him to barge into Guru Dutt's office pretending to be a drunkard and create a scene—an impromptu audition that won him the role in Baazi. Badruddin was later given the screen name Johnny Walker. While Baazi did not fully align with Sahni's politics and worldview at the time, he nonetheless infused the script with elements of social justice, realism, and the destitution he had explored during his time with IPTA.

Around the same time, Zia Sarhadi's Hum Log was released alongside Guru Dutt's Baazi. Sarhadi had seen Sahni perform in an IPTA play called Sadak Ke Kinare and had originally written the story of Hum Log for K. Asif. However, when he narrated it to Chandulal Shah of Ranjit Studio, Shah was so impressed that he took the project from Asif and handed it to Sarhadi to direct. Although Dilip Kumar was the original choice for the lead role, it ultimately went to Balraj Sahni. Initially unsure of himself, Sahni later experienced a moment of clarity that gave him the conviction to carry the part with confidence.

Balraj Sahni was at the beach with his children when Asit Sen, Bimal Roy's assistant, approached him with a message: Roy wanted to meet him. Sahni dressed up for the occasion in his finest London suit, powdered his face, and went to see Bimal Roy. However, Roy was disappointed—Sahni's appearance didn't match the image he had in mind for the protagonist of his next film. Hrishikesh Mukherjee narrated the story to him. After hearing the narration of Do Bigha Zamin, Sahni went to Jogeshwari, a slum inhabited by north Indian buffalo herders and tenders. He observed their gait, speech, posture, and how they wore their turbans. This keen observation helped him prepare for the role of a rickshaw puller in Do Bigha Zamin. He insisted on doing his own makeup and choosing his costume. When he finally presented himself to Roy, he had completely transformed into Shambhu Mahato, the peasant. Roy was deeply impressed, and this transformation added a great degree of authenticity to Sahni's performance.

In one scene, Sahni's character had to fall at the feet of the zamindar, played by Murad, and beg him to spare his land. Unbeknownst to Sahni, Bimal Roy had instructed Murad to suddenly jerk his feet away. Murad ended up kicking Sahni hard in the face. Shocked and humiliated, Sahni began sobbing in anger. Murad immediately rushed to embrace him and explained the situation.

Amiya Chakravarty's Seema was acclaimed for its performances and plot. In the same year (1955), he was also cast in Garm Coat, the first film of Cine Co-operative, a consortium consisting of Balraj Sahni, Rajinder Singh Bedi and Geeta Bali, among others. It was an adaptation of Nikolai Gogol's short story titledThe Overcoat.

Lal Batti (1957) was another film produced by Cine Co-operative, and the only film Balraj Sahni ever directed. He was drawn to the theme of the film becaue of his deep interest in social realism and a passion to tell stories of common people. Balraj co-directed the film with Krishan Chopra. The film didn't work and the consortium had to be shut down. Sahni never directed another film after this. In 1957, he lent his voice to Hrishikesh Mukherjee's directorial debut Musafir.

For Balraj, who would eventually find great success playing patriarchs, Bhabi (1957) was one of his early outings as the family elder. Sone Ki Chidiya, produced by Ismat Chughtai and directed by her husband Shahid Latif, was based on a short story written by her. It featured a poem recited by Kaifi Azmi, with Balraj Sahni lip-syncing to the words. The poem, “Aaj ki raat bahut garm hawa chalti hai,” would later foreshadow Sahni's swansong, the film Garm Hawa (1973).

Anuradha was Balraj Sahni's first acting role in a Hrishikesh Mukherjee film. It also marked the debut of Leela Naidu, a former Miss India pageant winner. Anuradha also marked Pandit Ravi Shankar's return to Hindi cinema as a composer, following his earlier work in Dharti Ke Lal and Neecha Nagar. Balraj Sahni's performance as the dutiful doctor received widespread acclaim.

Balraj went on a tour of Pakistan and visited his childhood home. Once back in India, he wrote his first major work in Punjabi, Mera Pakistani Safarnama. During this time, he was contributing book reviews to Bombay's Ranjit magazine, and was sending his Punjabi articles and poems to Preet Lari in Punjab and Arsi in Delhi.

Bimal Roy's Kabuliwala (1961) was his tribute to Rabindranath Tagore. Sahni, who always felt a deep connection to Tagore, found an opportunity to indluge in it once again. The film, based on Tagore's short story, explores the bond between an Afghan dry-fruit seller and a little girl. To prepare for the role, Sahni disappeared from home a couple of weeks before shooting and took up accommodation in a colony of Pathans in Sion. He learned their language, studied their demeanour and body language, and poured it all into his portrayal of Abdul Rahman Khan.

Anpadh (1962) was an attempt to spread the message of educating the girl child. In the film, Sahni appeared in a rare negative role. He played the heroine's brother, who loved her deeply but did not see the value in giving her an education — in fact, he became an obstacle to it. Jawaharlal Nehru, the then Prime Minister, was a strong proponent of women's education, and the film was designed to promote that message.

The activities of IPTA had cooled down since 1950, but Balraj craved the stage. So he created an amateur theatre group named 'The Juhu Art Theatre', comprising of himself, his wife Santosh, actors Nitin Sethi, Mohan Sharma and several others. In 1963, Juhu Art Theatre staged a production of Kabuliwala, where Balraj reprised his role from the screen.

Many artists affiliated with communism felt betrayed when China waged war against India. Though they had sympathised with China's brand of communism, the conflict left them disillusioned. As a result, several left-leaning actors chose to participate in Chetan Anand's Haqeeqat (1964). Balraj Sahni, an avowed leftist, was among them, along with Kaifi Azmi, M.S. Sathyu, and Shaukat Kaifi. Waqt (1965), one of Hindi cinema's first popular multi-starrers, featured Balraj Sahni as the patriarch of a family that disintegrates and finds each other by the end of the film. It featured Balraj Sahni's most well known appearance in a film song: "Ae meri zohra-jabeen".

Sunghursh was an adaptation of Mahasweta Devi's Layli Admaner Aayna, a Bengali novella about a family of thugs in 19th-century Benares. Sahni's role had shades of grey and placed him opposite a young Sanjeev Kumar, while also reuniting him with Dilip Kumar after Hulchul (1951).

Raj Khosla's Do Raaste cast Balraj Sahni as the family patriarch, with Rajesh Khanna and Prem Chopra playing his brothers. This was one of two films released in 1969, along with Aradhana, that elevated Rajesh Khanna to superstardom. By this time, Balraj was frequently playing father figures who were pivotal to the story and screenplay.

IPTA was showing signs of revival and Balraj was welcomed back into the fold. In 1969, he famously appeared as Mirza Ghalib in IPTA's 'Akhri Shama'. M.S. Sathyu directed it and the dialogues were written by Kaifi Azmi. Around the same time, Balraj wrote 'Mera Roosi Safarnama' an account of his travel to the USSR.

In Paraya Dhan (1971), Balraj Sahni once again plays a grey character who has shady past that his family is not aware of. He plays a hitman who leaves the world of crime to raise his daughte

Back in 1958, Kaifi Azmi had recited his poem “Makan” as the voice of Balraj Sahni in a film called Sone ki Chidiya, which began with the refain "Aaj ki raat bahut garm hawa chalti hai”. The film was written by Ismat Chugtai. 15 years later, based on an incomplete short story by Chugtai, M.S. Sathyu made the film 'Garm Hava' (1973) with Balraj Sahni as the central protagonist, a shoe manufacturer in Agra faced with the dilemma of partition. Was he to leave his beloved country of birth for a promised land, or just stay back. Amidst the turmoil of communal riots and hate-mongering, the aging patriarch has to endure the separation of a daughter who commits suicide. In real life, Balraj had just lost his own daughter Shabnam to a brain clot. In a poignant instance of art imitating life, Balraj's character finds his daughter dead, and the actor had to convey the pain with restraint, without wasting any words. Balraj Sahni's act in Garm Hava is considered one of the very best performances in his illstrious career.

Balraj Sahni passed away from a heart attack on 13 April 1973 in Bombay, just a day after finishing his work on Garam Hawa. His death was also preceded by the emotional trauma of losing his daughter the previous year. His lifelong commitment to social realism and emotive performances in landmark films redefined Indian cinema's portrayal of the common man, and his legacy continues to inspire generations of artists and storytellers.