He was born in Bhowanipore in Calcutta to Durgadas De and Trailokyamohini Devi. His family, originally from Uttar Rarh in Bengal, belonged to the newly emerging business community of Calcutta. From his memoir, the only written document available on the history of his family, his forefathers, especially his grandfather, Shyamsundar De, worked for a number of mercantile houses of the city, and for sometime owned quite a few houses in Bhowanipore. His father was a student of Hare School and Hindu College, Calcutta, (1847-1849), and later worked for Raja Dakshinaranjan Mukherjee, who took him to Lucknow. On his mother's side, which by his own admission was more distinguished than his father's, one of his ancestors in the late eighteenth century was Raja Manik Ram Bose, an agent of the Nawabs of Oudh. Also, on his mother's side he was a great-nephew of Peary Charan Sarkar, to whom he was close.
In the initial years he was admitted to the Hare School of which Sarkar was a headmaster. While still in Calcutta, young Brajendranath was greatly influenced by the personality of the radical journalist Harish Chandra Mukherjee who lived in the same neighbourhood as his family. In 1863, at the age of 11, he and his younger brother, Siddheshwar, travelled to Lucknow with their parents. From his account, in his memoir, of his family's standard of living in Lucknow, it appears that the family, which stayed in Kaiserbagh was 'passing rich'. After completing his remaining few years of schooling there, he joined Canning College, Lucknow, where he completed his B.A. degree, coming first class sixth. There he learnt Arabic under the guidance of Syed Hussain Bilgrami. He completed his M.A. (Honours) degree at the University of Calcutta in 1871, ranking first class second in the university. In 1872, with funds raised by his father and more importantly by Raja Dakshinaranjan Mukherjee, and also with the encouragement of Sarkar, he went to England, and joined University College, London. There he appeared for the Open Competitive Service Examination which he successfully took in 1873, becoming the eighth Indian and sixth Bengali to join the Indian Civil Service. He was also called to the Bar by the Honourable Society of the Middle Temple. In 1875, he was one of only two students to be awarded the Boden Sanskrit Scholarship which he held at St Mary Hall, Oxford for one year, and was a student of Professor Sir Monier Monier Williams. As an Indian student in England he became friendly with the eminent nationalist leader Ananda Mohan Bose, and Bolinarayan Borah, later a successful government official, and son-in-law of Romesh Dutt.
Upon return from England, as a fresh recruit in the ICS, he rented a house in Jorashanko, in North Calcutta where his family, including his parents and wife, Nagendra Nandini, stayed from 1875 to 1877. After his father passed away in 1877, he moved with his family to the districts, although his mother divided her time between her elder son's official homes and her husband's ancestral home in Bhowanipore. He served in various bureaucratic capacities in Bengal, Behar and Orissa.
Like his father, who used to visit the Brahmo Samaj, he too adhered to the Brahmo faith. Slightly later in his life, he joined the Sammilani Samaj, a branch of the Brahmo Samaj, which is still based in Bhowanipore, Calcutta. There is, however, a pleasant description in his fourth son-in-law, Gurusaday Dutt's book entitled A Woman of India: Being the Life of Saroj Nalini Dutt, of his family members, especially his mother, who was a devout Hindu, and also his wife and children performing their daily prayers in the evening by the river Hooghly at his garden-house in Bandel.
His first posting in the civil service was as Assistant Magistrate of Arrah in Behar. The Ilbert Bill Controversy of 1883 took place when he was a Joint Magistrate of Hooghly. When asked to comment on the nature of the bill, he supported the recommendations for increase in the Indian magistrates' powers. This gained him the contempt of his then Divisional Commissioner, John Beames, esq., who in the Aitchison Committee suggested that persons of Brajendranath's nationality, not calibre, should neither be allowed to draw a salary equal to that of the British civilians, nor should they be allowed to sit in the presence of their British counterparts in the same room. The members of the committee showed disrgard for Beames' views and asked the latter to leave the room while Brajendranath was asked to make his deposition before the committee.
He was elected the first Indian Chairman of the Hooghly Municipal Corporation at the end of the nineteenth century and contributed to its civic upliftment, such as the improvement of the water works there. He also contributed significantly to the development of Uttarpara, and became a friend of its zamindar, Raja Peary Chand Mukherjee. In British official circles he was known to have taken the side of the moderate nationalists. He was Magistrate and Collector of a number of districts in Bengal and Orissa, namely Faridpore, Khulna, Hooghly, and Balasore. As the District Officer of Balasore, he worked closely with his Commissioner, Romesh Dutt, esq., ICS, one of the stalwarts of moderate nationalism.
As Magistrate and Collector of Balasore, he was appointed as Assistant Superintendent of the Tributary Mahals of Mayurbhanj, Keonjhar and Nilgiri He was particularly involved in the administration of Mayurbhanj, where both he and Dutt had to deal with the Dowager Maharani and her minor grandson, Maharaja Sri Ram Chandra Bhanj Deo's marriage alliance. He was also in close contact with the zamindars Dharbhanga and Bhabua in Behar. He writes that the chief of the zamindars of Bhabua was not half as wealthy as the other landlords of the region, but was still considered the first amongst equals and was held in very high esteem. He was one of the principal functionaries in the Court of Wards, when Maharaja Bijoy Chand Mehtab of Burdwan, then a minor was placed under its supervision. During this period he worked in close association with the Maharaja's father, the Dewan of Burdwan, Raja Banbehari Kapoor.
He was appointed as (Acting) Commissioner of Burdwan thrice in 1905. Due to his pro-nationalist sentiments and decision to visit a number of Swadeshi Bazaars in the division in that year, he was severely criticised by his British colleagues in the civil service. This act of patriotism, however, won him high praise in nationalist circles, especially from Congress leaders, such as Surendranath Banerjee, and also from his Indian colleagues in the civil service. The Bengalee, the nationalist daily edited by S.N.Banerjee wrote:
What are the sovereign recipes for the unrest that Mr.De had? A little intuitive sympathy with his countrymen, born of first-hand knowledge of their desires, their requirements, their character and their temperament, and as the Pioneer says there was no trouble in Hughli. The Pioneer's testimony to the efficacy of sympathy in the governance of men is complete.
Even the Pioneer, which was a pro-British newspaper, supported the view that De was an exceptionally able and patriotic administrator, committed to the maintenance of law and order in his district. It wrote: "If it had been possible to multiply Mr.De sufficiently, there would have been no trouble in Bengal". The Indian Opnion added: "... but these are men who glide out of the service unnoticed while the person who is chiefly responsible for the mischief probably makes his exit under salutes, in a coat covered with ribbons and stars."
As the Collector of Hooghly, he started a club, called the Duke Club in the district exclusively for Indians. His decision to start a club only for the Indians was promted by the British refusal to allow any Indian entry into any one of their clubs. Once his commissioner reportedly told him not to even entertain any thought of wanting to join one of the British clubs in his district. Social prejudices of his British colleagues towards the Indian members of the covenanted civil services were responsible for his steadfast support for the other Indians of his district, which won him many friends and rich accolades throughout his career, especially at the time of his retirement. The Indian Daily News reported that:
Both Hindus and Mohammadans, headed by Raja Peary Mohon Mukherjee, C.S.I. and by Nawabzada Sayid Ashrafuddin Ahmed Khan Bahadur are going to give Mr. and Mrs. De two farewells... It must be noted that with the exception of Messrs. Faulder, Inglis, Duke and Maddox, no civilian was more popular in Hooghly District than Mr.B.De.
He retired from the civil service in 1910. After retirement he became a Vice President of the Asiatic Society of Bengal, and translated and edited in two volumes Nizamuddin Ahmad's Tabaqat-i-Akbari. The third volume, which he had left fully prepared, was published posthumously by Dr.Hidayat Hosain. This book gives a general history of India from the Mohammadan conquest up to 1594. In retirement, he was also appointed as a Member of the Calcutta Improvement Trust.