Many portraits of Martineau, including one painted by George Frederick Watts, are held at London's National Portrait Gallery. In 2014, the gallery revealed that its patron, Catherine, Duchess of Cambridge, was related to Martineau. The Duchess' great-great-grandfather, Francis Martineau Lupton, was Dr James Martineau's grandnephew. The gallery also holds written correspondence between Martineau and Poet Laureate, Alfred, Lord Tennyson - who records that he "regarded Martineau as the master mind of all the remarkable company with whom he engaged". William Ewart Gladstone said of Martineau; "he is beyond question the greatest of living thinkers".
The seventh of eight children, James Martineau was born in Norwich, England, where his father Thomas (1764–1826) was a cloth manufacturer and merchant. His mother, Elizabeth Rankin, was the eldest daughter of a sugar refiner and grocer. The Martineau family were descended from Gaston Martineau, a Huguenot surgeon and refugee, who married Marie Pierre in 1693, and settled in Norwich. His son and grandson — respectively the great-grandfather and grandfather of James Martineau — were surgeons in the same city. Many of the family were active in Unitarian causes, so much so that a room in Essex Hall, the headquarters of British Unitarianism, was eventually named after them. Branches of the Martineau family in Norwich, Birmingham and London were socially and politically prominent Unitarians; other elite Unitarian families in Birmingham were the Kenricks, Nettlefolds and the Chamberlains, with much intermarriage between these families taking place. Essex Hall held a statue of Martineau. His niece, Frances Lupton, who was close to his sister Harriet, had worked to open up educational opportunities for women.
James was educated at Norwich Grammar School where he was a school-fellow with George Borrow under Edward Valpy, as good a scholar as his better-known brother Richard, but proved too sensitive for school. He was sent to Bristol to the private academy of Dr. Lant Carpenter, under whom he studied for two years. On leaving he was apprenticed to a civil engineer at Derby, where he acquired "a store of exclusively scientific conceptions," but also began to look to religion for mental stimulation.