Sheikh Mujib was elected General Secretary of Islamia College Students Union in 1946. The next year Bangobondhu obtained Bachelor of Arts degree from Islamia College under Calcutta University. When communal riots broke out in the wake of the partition of India and the birth of Pakistan, Bangobondhu played a pioneering role in protecting Muslims and trying to contain the violence
In 1948, Bangobondhu took admission in he Law Department of Dhaka University. He founded the Muslim Students League on 4 January. He rose in spontaneous protest on 23rd February when Prime Minister Khwaja Nazimuddin in his speech at the Legislative Assembly declared : "The people of East Pakistan will accept Urdu as their state language".
Khwaja Nazimuddin's remarks touched off a storm of protest across the country. Sheikh Mujib immediately plunged into hectic activities to build a strong movement against the Muslim League's premeditated, heinous design to make Urdu the only state language of Pakistan. He established contracts with students and political leaders. On 2 March, a meeting of the workers of different political parties was held to chart the course of the movement against the Muslim League on the language issue. The meeting held at Fazlul Huq Hall approved desolation placed by Bangobondhu to form an All-Part State League age Action Council.
The Action Council called for a general strike on 11 March to register its protest against the conspiracy of the Muslim League against Bangla. On 11 March, Bangobondhu was arrested along with some colleagues while the were holding a demonstration in front of the Secretariat building. The student community of the country rose in protest following the arrest of Bangobondhu. In the face of the strong student movement the Muslim League government was forced to release Bangobondhu and other student leaders on 15 march. Following his release, the All-Party State Language Action Council held a public rally at Dhaka University Bat Tala on 16 March. Bangobondhu presided over the rally, which were soon sets upon by the police.
To protest the police action Bangobondhu immediately announced a countrywide student strike for 17 March. Later, on 19 May, Bangobondhu led a movement in support of the Dhaka University Class Four employees struggling to redress the injustice done to them by their employers. Mujib was arrested again on 11 September.
Sheikh Mujib was released from jail on 21 January, 1949. Bangobondhu extended his support to a strike called by the Class Four employees of Dhaka University to press home their various demands. The university authorities illogically imposed a fine on him for leading the movement of the employees. He rejected the unjust order. Eventually, the anti-Muslim League candidate Shamsul Huq won a by-election in Tangail on 26 April, Mujib was arrested for staging a sit-in strike before the vice-chancellor's residence. When the East Pakistan Awami Muslim League was formed on 23 June, Bangobondhu was elected its joint secretary despite his incarceration. He was released in late June. Immediately after his release, he began organizing an agitation against the prevailing food crisis. In September he was detained for violating Section 144. Later, however, he was freed.
He raised the demand for Chief Minister Nurul Amin's resignation at a meeting of the Awami Muslim League in October. Immediately afterward, he was arrested again alone with Moulana Abdul Hamid Khan Bhashani for leading a delegation to Liaquat Ali Khan. That was towards the end of October.
On the first of January, 1950, the Awami Muslim League brought out an anti-famine procession in Dhaka on the occasion of Pakistan's Prime Minister Liaquat Ali Khan's visit to the province. Once again Bangobondhu was arrested and jailed, this time for two years, for leading the demonstration.
On 26 January, 1952, Khwaja Nazimuddin declared that Urdu would be the state language of Pakistan. Though still in jail, Bangobondhu managed to play a leading role in organizing a protest against this announcement. From prison he sent out a call to the State Language Action Council to obverse 21 February as Demand Day for releasing political prisoners and making Bangla the state language. He began a hunger strike on 14 February. On 21 February the student community violated Section 144 and brought out a procession in Dhaka to demand the recognition of Bangla as the state language. Police opened fire, killing in the process Salam, Barkat, Rafique, Jabbar and Shafiur, who thus became martyrs of the Language Movement. In a statement from jail, Bangobondhu condemned the police firing and registered his strong protest. He was on hunger strike for 13 consecutive days. He was moved from Dhaka central jail to Faridpur Jail to prevent him from making contact with the organizers of the movement. He was released from jail on 26 February.