Educated at the University of Notre Dame and the University of Oxford, he was called to the bar of England and Wales in Lincoln's Inn in 1959. Hossain enrolled as an advocate in the High Court of East Pakistan. He worked on cases with prominent Pakistani lawyers, including Huseyn Shaheed Suhrawardy and Mahmud Ali Kasuri, early in his legal career. Hossain often worked on "habeas corpus" cases during the Ayub Khan regime. Between 1961 and 1968, he taught law at Dhaka University. Hossain was the legal counsel of Sheikh Mujibur Rahman and the Awami League during the Agartala Conspiracy Case, and was elected vice-chairman of the Pakistan Bar Council in 1970. In 1971, he was part of the League's negotiation team for the transfer of power after the 1970 general election. Hossain was imprisoned in West Pakistan with Sheikh Mujibur Rahman during the war of independence that transformed East Pakistan into Bangladesh.