He started his practice as a barrister from Bombay High Court in 1915. He was also the editor of Indian Law Reports for a brief time. In 1930, he ascended to the position of Acting Judge of Bombay High Court. After that, from 1931 to 1933, he was made the additional judge of the same court. He should have become the Chief Justice of this court upon the incumbent judge's retirement. However, back in those days, the Britishers favoured their kind, leaving Justice. Kania is waiting to be promoted.
During his time in the Supreme Court, J. Kania had authored 36 judgments and earned 17 citations. He thought that the Supreme Court was not authorized to question the Parliament's wisdom. This view of Justice Kania has been further explained in one of his notable judgments of A.K. Gopalan v. State of Madras, 1950. However, this judgment has been criticized as bearing colonial jurisprudence and not in consonance with India's new Constitution.
The promotion came in 1946 when he became the associate judge of the Federal Court. Finally, he was appointed as the first Chief Justice of India in 1950 after India got its independence.Sadly, he died while serving in the office within one year of service itself,due to cardiac arrest in 1951 at 61.