Allah Wasai began to sing at the age of five or six years old and showed a keen interest in a range of styles, including traditional folk and popular theatre. Realising her potential for singing, her mother sent her to receive early training in classical singing under Ustad Bade Ghulam Ali Khan who was also a native of Kasur. He instructed her in the traditions of the Patiala Gharana of Hindustani classical music and the classical forms of thumri, dhrupad, and khyal. At the age of nine, Wasai drew the attention of Punjabi musician Ghulam Ahmed Chishti, who would later introduce her to stage in Lahore. He composed some ghazals, naats and folk songs for her to perform, although she was more keen in breaking into acting or playback singing. Once her vocational training finished, Wasai pursued a career in singing alongside her sisters in Lahore and would usually take part in the live song and dance performances prior to screenings of films in film theatres.

The family moved to Calcutta (now Kolkata) in hope of developing the movie careers of Wasai and her sisters. During their stay in Calcutta, the renowned singer Mukhtar Begum, encouraged Wasai and her two older sisters to join film companies and recommended them to various producers. She also recommended them to her husband, Agha Hashar Kashmiri, who owned a maidan theatre (a tented theatre to accommodate large audiences). It was here that Wasai received the stage name Baby Noor Jehan. Her older sisters were offered jobs with one of the Seth Sukh Karnani companies, Indira Movietone and they went on to be known as the Punjab Mail. Wasai would later adopt Mukhtar Begum's way of performance and sari attire.

In 1935, K.D. Mehra directed Pind di Kudhi in which Jehan acted along with her sisters.She next acted in a film called Missar Ka Sitara (1936) by the same company and sang in it for music composer, Damodar Sharma.Baby Noor Jehan also played the child role of Heer in the film Heer-Sayyal (1937). After a few years in Calcutta, Noor Jehan returned to Lahore in 1938. In 1939, Ghulam Hairder composed songs for Jehan which led to her early popularity. She then recorded her first song Shala Jawaniyan Mane for Dalsukh M. Pancholi's movie Gul Bakavli.

Prior to Khandaan Jehan was cast as a child artist. It was in 1942 that she played the main lead opposite Pran. Khandaan's success saw her shifting to Bombay (now Mumbai), where she shared melodies with Shanta Apte in Duhai (1943). It was in this film that Noor Jehan lent her voice for the second time, to another actress named Husn Bano. In 1945 Jehan player the lead role, alongside Lata Mangeshkar and Asha Bhosle, in the movie Badi Maa.

In 1945, she achieved a milestone, when she sung a Qawwali with Zohrabai Ambalewali and Amirbai Karnataki which was "Aahen Na Bhareen Shikave Na Kiye". This was the first ever Qawwali recorded in female voices in South Asian films.

Noor Jehan's last film in India was Mirza Sahibaan (1947) which starred Prithviraj Kapoor's brother Trilok Kapoor. Noor Jehan sang 127 songs in Indian films and the number of talking films she made from 1932 to 1947 was 69. The number of silents was 12. Fifty-five of her films were made in Bombay, eight in Calcutta, five in Lahore and one in Rangoon (now Yangon), Burma.