In 1953, he was made the party's general secretary, and elected to the East Bengal Legislative Assembly on a United Front coalition ticket in 1954. Serving briefly as the minister for agriculture during A. K. Fazlul Huq's government, Mujib was briefly arrested for organising a protest of the central government's decision to dismiss the United Front ministry.
He was elected to the second Constituent Assembly of Pakistan and served from 1955 to 1958. The government proposed to dissolve the provinces in favour of an amalgamation of the western provinces of the Dominion of Pakistan in a plan called One Unit; at the same time the central government would be strengthened. Under One Unit, the western provinces were merged as West Pakistan during the creation of the Islamic Republic of Pakistan in 1956. That year East Bengal was renamed as East Pakistan as part of One Unit at the same time. Mujib demanded that the Bengali people's ethnic identity be respected and that a popular verdict should decide the question of naming and of official language:
"Sir [President of the Constituent Assembly], you will see that they want to place the word "East Pakistan" instead of "East Bengal." We had demanded so many times that you should use Bengal instead of Pakistan. The word "Bengal" has a history, has a tradition of its own. You can change it only after the people have been consulted. So far as the question of One Unit is concerned it can come in the constitution. Why do you want it to be taken up just now? What about the state language, Bengali? We will be prepared to consider one-unit with all these things. So I appeal to my friends on that side to allow the people to give their verdict in any way, in the form of referendum or in the form of plebiscite."
n 1956, Mujib entered a second coalition government as minister of industries, commerce, labour, anti-corruption and village aid. He resigned in 1957 to work full-time for the party organisation.
In 1958 General Ayub Khan suspended the constitution and imposed martial law. Mujib was arrested for organising resistance and imprisoned till 1961. After his release from prison, Mujib started organising an underground political body called the Swadhin Bangal Biplobi Parishad (Free Bangla Revolutionary Council), comprising student leaders, to oppose the regime of Ayub Khan. They worked for increased political power for Bengalis and the independence of East Pakistan. He was briefly arrested again in 1962 for organising protests.