In 1929, Mujib entered into class three at Gopalganj Public School, and two years later, class four at Madaripur Islamia High School. However, Mujib was withdrawn from school in 1934 to undergo eye surgery, and returned to school only after four years, owing to the severity of the surgery and slow recovery. At the age of eighteen, Mujib married Sheikh Fazilatunnesa Mujib. Together they had two daughters—Sheikh Hasina and Sheikh Rehana—and three sons—Sheikh Kamal, Sheikh Jamal, and Sheikh Rasel.
Mujib became politically active when he joined the All India Muslim Students Federation in 1940. He enrolled at the Islamia College (now Maulana Azad College), a well-respected college affiliated to the University of Calcutta to study law, and entered student politics there.
He joined the Bengal Muslim League in 1943. During this period, Mujib worked actively for the League's cause of a separate Muslim state of Pakistan, and in 1946 he went on to became general secretary of the Islamia College Students Union. M. Bhaskaran Nair describes that Rahman "emerged as the most powerful man in the party" because of his close proximity to Huseyn Shaheed Suhrawardy.
After obtaining his degree in 1947, Mujib was one of the Muslim politicians working under Suhrawardy during the communal violence that broke out in Calcutta, in 1946, just before the partition of India.
After the Partition of India, Rahman chose to stay in the newly created Pakistan. On his return to what became known as East Pakistan, he enrolled in the University of Dhaka to study law and founded the East Pakistan Muslim Students' League. He became one of the most prominent student political leaders in the province. During these years, Mujib developed an affinity for socialism as the solution to mass poverty, unemployment and poor living conditions. On 26 January 1949 the government announced that Urdu would be the only official state language of Pakistan, although Bengali was the majority language in East Pakistan. Though still in jail, Mujib encouraged fellow activist groups to launch strikes and protests; he undertook a hunger strike for 13 days.
Following the declaration of Muhammad Ali Jinnah and the province chief minister Khwaja Nazimuddin in 1948 that the people of East Bengal would have to adopt Urdu as the state language, protests broke out amongst the population. Mujib led the Muslim Students' League in organising strikes and protests, and was arrested along with Khaleque Nawaz Khan and Shamsul Haque by police on 11 March. The sustained protest from students and political activists led to the immediate release of Mujib and the others. Mujib was expelled from the university and arrested again in 1949 for attempting to organise the menial and clerical staff in an agitation over workers' rights.