Janaki Agnes Penelope Majumdar was the daughter of the Hemangini and W. C. Bonnerjee, the first president of the Indian National Congress in December 1885. Born in Calcutta, in June 1886, Janaki, her mother and siblings settled in England from 1888. They soon moved into a house they named 'Kidderpore' in Croydon. Janaki spent 1893-5 back in India and then returned to England and went to Croydon High School for Girls.
She studied at Newnham College, Cambridge, in 1904, and was the first Indian woman to receive a degree in Natural Sciences. Following the death of her father in 1906, 'Kidderpore' was sold. Janaki began a teacher's training course at the London Day Training College in 1907 and did voluntary work at the Charity Organization Society's Newington Branch. In 1908, she returned to Calcutta with her mother and met P. K. Majumdar. He had studied at Birmingham University and trained as a barrister in London. They were married in 1909 and lived in Calcutta. She returned to London following her husband's death in 1947.
In 1935, Janaki wrote a family memoir about her childhood, her father and her husband, with a major emphasis on her mother, Hemangini. It tells of a South Asian family living in England in the late nineteenth and early twentieth centuries. This memoir, Family History, was edited by Antoinette Burton and published in 2003.