Hossain was appointed as the first law minister of the newly formed country. One of his chief responsibilities was to frame a constitution. He was chairman of the Constitution Drafting Committee in the Constituent Assembly of Bangladesh. He was 35 years of age at the time, making him one of the youngest constitution drafters in the world. Hossain was credited for leading the successful completion of the constitution's drafting process. The Constitution of Bangladesh was approved on 4 November 1972 and came into force on 16 December 1971. The constitution included a bill of rights and professed parliamentary government (scrapped in 1975 and restored in 1991).