The British East India Company annexed Bengal in 1765, and Assam in 1838.
In 1824 Assam was first occupied by British forces following the First Anglo-Burmese War and on 24 February 1826 it was ceded to Britain by Burma.
Between 1826 and 1832 Assam was made part of Bengal under the Bengal Presidency.
From 1832 to October 1838 the Assam princely state was restored in Upper Assam while the British ruled in Lower Assam. Purandar Singha was allowed to rule as king of Upper Assam in 1833, but after that brief period Assam was annexed to Bengal by the British.
In 1873 British political control was imposed on western Naga communities.
As early as 1868, British administrators saw the need for an independent administration in the eastern portion of the Bengal Presidency.
On 6 February 1874 Assam, including Sylhet, was severed from Bengal to form the Assam Chief-Commissionership, also known as the 'North-East Frontier'. Shillong was chosen as the capital of the Non-Regulation Province of Assam in September 1874.
The Lushai Hills were transferred to Assam in 1897. The new Commissionership included the five districts of Assam proper (Kamrup, Nagaon, Darrang, Sibsagar and Lakhimpur), Khasi-Jaintia Hills, Garo Hills, Naga Hills, Goalpara and Sylhet-Cachar comprising about 54,100 sq. miles. Cooch Behar a historical part of Assam, was left out. But during the partition of Bengal in 1905, the Assam territory was incorporated into the new Province of East Bengal and Assam.
The province of East Bengal and Assam was annulled in 1911 following a sustained mass protest and on 1 April 1912 the two parts of Bengal were reunited. A new partition based on language followed, Oriya and Assamese areas were separated to form new administrative units: Bihar and Orissa Province was created to the west, and Assam Province to the east.
The Government of India Act 1935 provided provincial autonomy and further enlarged the elected provincial legislature to 108 elected members. In 1937 elections were held for the newly created Assam Legislative Assembly established in Shillong. The Indian National Congress had the maximum number of seats with 38 members but declined to form a government. Therefore, Sir Syed Muhammad Saadulla was invited to form the government. Saadulla's government resigned in September, 1938 and the Governor then invited Gopinath Bordoloi. Bordoloi's cabinet included future President of India Fakhruddin Ali Ahmed.
In 1939, all of the Congress ministries in British Indian provinces resigned and a new government under Saadulla was formed. Sir Syed Muhammad Saadulla remained as the Premier or Chief Minister of Assam till 1946 barring a brief period of Governor Rule.
During the Japanese invasion of India in 1944, some areas of Assam Province, including the Naga Hills district and part of the Manipur princely state, were occupied by Japanese forces between mid March and July. When fresh elections to the provincial legislatures were called in 1946, the Congress won a majority in Assam and Bordoloi was again the Chief Minister. Prior to the Independence of India, on 1 April 1946, Assam Province was granted self-rule.
The British partitioned Bengal again in 1947, making Muslim-majority districts of East Bengal a part of the Dominion of Pakistan (East Pakistan). Most of the colonial Assam Province became a part of the Union of India, and was eventually divided into several states, including Nagaland, Mizoram, Meghalaya, Assam proper, Tripura and Manipur. Bordoloi continued as the Chief Minister even after India's independence in 1947.
Assam's Surma Valley had Muslim-majority population. On the eve of partition, hectic activities intensified by the Muslim League as well Congress with the former having an edge. A referendum had been proposed for Sylhet District. Mazumdar along with Basanta Kumar Das (then Home Minister of Assam) travelled throughout the valley organising the Congress and addressing meetings educating the masses about the outcome of partition on the basis of religion.
On 20 February 1947, Moulvi Mazumdar inaugurated a convention – Assam Nationalist Muslim's Convention at Silchar. There after another big meeting was held at Silchar on 8 June 1947. Both the meetings, which were attended by a large section of Muslims paid dividend. He was also among the few who were instrumental in retaining the Barak Valley region of Assam, especially Karimganj with India. Mazumdar was the leader of the delegation that pleaded before the Radcliffe Commission that ensured that a part of Sylhet remains with India despite being Muslim-majority (present Karimganj district).
The entire eastern India was swept by violence just after India's partition and independence on 15 August 1947, scores of Hindus fled the newly created East Pakistan for India, and Muslims fled Assam for East Pakistan. A large number of people lost their lives owing to violence, which resurfaced with more ferocity in 1950.
Mazumdar, the only Muslim in the cabinet, along with his cabinet and party colleagues took up responsibility for the safety of both Hindus and Muslims in Karimganj, touring affected areas and arranging camps and rehabilitation for the refugees, organizing supplies and security.
During the liberation war in 1970-71, he was in charge of relief-&-rehabilitation of the thousands of refugees who fled the then East Pakistan to Assam including Karimganj. East Pakistan, gained independence as the country of Bangladesh in 1971.
Shillong was the summer capital of the undivided Eastern Bengal and Assam. There were 4 administrative divisions in the province, including the Assam Valley Division, Chittagong Division, Dacca Division, Rajshahi Division and the Surma Valley and Hill Districts Division. There were a total of 30 districts, including Dacca, Mymensingh, Faridpur, Backergunje, Tippera, Noakhali, Chittagong, the Chittagong Hill Tracts, Rajshahi, Dinajpur, Jalpaiguri, Rangpur, Bogra, Pabna, Malda, Goalpara, Kamrup, Darrang, Nowgong, Sibsagar, Lakhimpur, Sylhet, Cachar, the Garo Hills, the Khasi and Jaintia Hills, the Naga Hills and the Lushai Hills.
Cooch Behar fell under the jurisdiction of Rajshahi Division, Manipur under the Assam Valley Division and Hill Tippera under Chittagong Division. The provincial government in Dacca also managed relations with Bhutan.
The population of Eastern Bengal and Assam was 30,961,459 in 1901. The densely populated districts in East Bengal and the Surma and Brahmaputra Valleys were home to Indo-Aryan ethnic groups, including the Bengalis (27,272,895) and the Assamese (1,349,784). Hill districts were home to a predominantly Tibeto-Burman population, including groups like the Chakmas, Mizos, Nagas, Garos and Bodos. There were 18,036,688 Muslims and 12,036,538 Hindus. The remainder included Buddhists, Christians and animists. With reference to the census in 1911, the population of Dhaka was 21% higher than that of 1906, when it was made the capital of the newly formed state.
Within its short lifespan, the Provincial Education Department promoted a significant expansion and improvement of higher education. Persian, Sanskrit, mathematics, history and algebra were among different disciplines introduced in the college level curriculum. Female colleges were established in each district. School enrollment increased by 20%. A committee was formed for the creation of the University of Dacca, which was established later in 1921, and came to be known as the Oxford of the East.
Eastern Bengal and Assam possessed one of the most fertile lands in the British Empire. The eastern Bengal delta was the rice basket of the Indian subcontinent. It produced 80% of the world's jute and dominated supply in the once thriving global jute trade. The Assam and Sylhet Valleys were home to the largest tea plantations in the world and became famous for producing high-quality Assam tea. The province was also a centre of the petroleum industry, due to crude oil production in Assam. The Port of Chittagong began to flourish in international trade and was connected to its hinterland by the Assam Bengal Railway. Shipbuilding was a major activity in coastal Bengal and catered to the British naval and merchant fleets. Dyeing industries were set up in several districts, particularly in Pabna and Dhaka.
The two main rail lines in Eastern Bengal and Assam were the Eastern Bengal Railway and the Assam Bengal Railway. The port city of Chittagong was the main rail terminus, as routes connected the interior hinterland with the main regional maritime gateway. Railways were vital for the export of tea, jute and petroleum. A number of new ferry services were introduced connecting Chittagong, Dhaka, Bogra, Dinajpur, Rangpur, Jalpaiguri, Maldah and Rajshahi. This improved communication network created a positive impact on the overall economy, boosting trade and commerce. Newly built highways connected the inaccessible areas of Assam and the Chittagong Hill Tracts. All district capitals were connected by an inter-district road network.