Sold his first screenplay, Naya Sansar, to Bombay Talkies in 1941, and with that very first attempt, ended up winning the BFJA (Bengal Film Journalists Association) Award for Best Screenplay. Abbas had become a member of the Progressive Writers' Association (PWA) and became one of the founding members of Indian People's Theatre Association (IPTA), as it took birth in 25 May 1943 at the premises of the Marwali School, Bombay. His affiliation with Indian People's Theatre Association (IPTA) in its early days further strengthened his nationalism, his Marxist leanings and his steadfast support of the working classes. He wrote two iconic plays for IPTA, Zubedia and Yeh Amrit Hai.

He directed IPTA's first film production, Dharti Ke Lal (1946), which was based on Krishan Chandar's novella Annadata and Bijon Bhattacharya's play Nabanna, both on the theme of Bengal famine which had killed millions of Indians. Made under the banner of IPTA, the film was noted for its use of non-professional actors alongside IPTA regulars like Balraj Sahni, Shombhu Mitra, Tripti Mitra and Zohra Sehgal.
Abbas also wrote the other film fuelled by his IPTA compatriots, Chetan Anand's Neecha Nagar which went on be honoured at the Cannes Film Festival.
Released around the same time, V. Shantaram's seminal film Dr Kotnis ki Amar Kahani was also written by Abbas.

Abbas launched his production banner called Naya Sansar in 1951. that year, he also visited China as part of an Indian delegation. Awara came out. When Abbas had originally conceived the story, he approached Mehboob Khan to direct it. But Mehboob wanted to cast Dilip Kumar in the lead, while Abbas wanted Raj Kapoor to do it. Raj's father Prithiviraj Kapoor was a fellow compatriot at IPTA. Raj had just made Aag and was on the lookout for a script - he came to know of Abbas' script and his preference for the lead role. Raj got in touch with Abbas and decided to make the film himself. The film heralded a long partnership, in which Abbas and Raj continued to make films that propagated the vision of a socialist India with strong egalitarian ideals.

Shree 420, written by Abbas, was released. The film explores his recurring themes of exploitation, class struggle, greed, and social decadence. Much like Awara, Shree 420 was an exploration of themes typical of Abbas' collaboration with Raj Kapoor. It also featured an Indian variant of Chaplin's tramp figure immortalised in his films. But Raj and Abbas brought in a desi flavour to this archetype which helped mirror the zeitgeist of the period.
After Dharti Ke Lal, Abbas once again collaborated with Sombhu Mitra on Jaagte Raho (1956). While Mitra was the actor in the former film and Abbas was in the director's seat, this time Sombhu was the director and Abbas the writer.
Abbas co-directed an Indo-Russian co-production named Pardesi aka Khozhdenie za tri morya (1957) about a 15th century Russian trader Afanasy Nikitin who had come to India. Oleg Strizhenov played the lead, and the film was bilingual: while the Hindi version was directed by Abbas, the Russian version was directed by Russian filmmaker Vasili Pronin.

Based on a story penned by himself titled One Thousand Nights on a Bed of Stones, Abbas made Shehar Aur Sapna (1963) depicting the struggles, hopes and dreams of pavement dwellers. It won the National Award for Best Feature Film.
He also wrote and directed Aasman Mahal (1965), about a Nawab (Prithviraj Kapoor) who fails to keep up with the times. Parallels Satyajit Ray's The Music Room/ Jalsaghar.
Bambai Raat Ki Baahon Mein, Abbas' crime thriller, was about a reporter who is suspected of a murder. The film depicted the decadence and the underbelly of modern urban life.

Abbas conceived the idea of his next film, Saat Hindustani, reinforcing the principles of unity and national integration by casting actors from various regions of the country to play characters from other regions. He cast Utpal Dutt, a Bengali, as a Punjabi Sardar, and Madhavan Nair, a Malayali, as a Bengali character. Amitabh Bachchan famously made his debut in this film, playing one of the seven 'Hindustanis' who set out to liberate Goa from Portuguese rule. The film won the Nargis Dutt Award for Best Feature Film on National Integration.

His film Chaar Sheher Ek Kahani (1970) came in the eye of a storm. It focused on the contrasting lives of the rich and poor in the four major cities of India- Delhi, Calcutta, Bombay and Madras. The Examining Committee of the Censor Board suggested the film to be restricted to adults due to a particular sequence depicting the red-light district of Bombay. But Abbas wanted a U certificate which would allow him to show the film without any cuts. The U certificate was then offered on one condition: that Abbas remove the sequence in question. Abbas refused and went to the courts, arguing that the imposition of pre-censorship violated his freedom of expression. In K.A. Abbas v. Union of India, the Supreme Court of India upheld the constitutionality of pre-censorship, while conceding the need for clear and unambiguous guidelines in such matters.
Wrote Mera Naam Joker (1970) came out. Unlike their early work, this film of Raj Kapoor/KA Abbas highlighted similar themes from an individualistic perspective, the journey of a circus clown. More than a straight social commentary, it accentuated the histrionics of Raj Kapoor, the actor.
Next, Abbas made the film Do Boond Pani (1971) about water scarcity in Rajasthan and the cost of progress. The film starred Jalal Agha and Simi Garewal in the lead and won the Nargis Dutt Award for Best Film on National Integration.

Wrote and appeared in an Indo-Soviet film called Black Elephant (1971) . The film was based on Abbas' short story for children. This was also the only time he appeared in a film.

This year, Abbas was associated with two important films which, like much of his work, highlighted societal issues and the human condition. He wrote Raj Kapoor's Bobby, which, while being a teenage love story, depicted the stark contrast between the lifestyles of the rich and the poor. Gulzar's Achanak was based on K.A. Abbas's short story The Thirteenth Victim, which explores a soldier's act of killing his wife and her lover, the futility of violence, and the complex nature of justice.

His film The Naxalites, a sort of spiritual successor to Dharti Ke Lal that even incorporates footage from the earlier film, depicted young people disillusioned by the state of post-independence India and the rampant exploitation around them. The film starred, among others, Smita Patil and Mithun Chakraborty.

Khwaja Ahmad Abbas passed away at the age of 72. He had written more than 70 books, countless articles and several popular columns, short stories and novels, directed 17 films and written at least 25 screenplays. His swansong Ek Aadmi starring Anupam Kher in the lead released a year later. And 4 years after his passing, when Henna was released, he was credited for story alongside V.P. Sathe.