Martineau was the son of Elizabeth Batty and Philip Martineau, a Master in Chancery. Through his mother, he was the grandson of Robert Batty, M.D. (1763–1849), physician and amateur painter.
Martineau attended Colfes school for a few years at the age of 15. He first trained as a lawyer and later entered the Royal Academy where he was awarded a silver medal. He studied under Pre-Raphaelite artist William Holman Hunt and once shared a studio with him. He died at the age of 43. In 1865, he married Maria Wheeler and had two children with her.
Robert Braithwaite Martineau, The Last Day in the Old Home (1862; Tate Gallery, London)
His most famous painting, The Last Day in the Old Home portrays shows the feckless squire after gambling away his familys inheritance. The man portrayed is Colonel John Leslie Toke (1839–1911) who was a friend of Martineau and was painted at his country home, Godinton House in Ashford, Kent. In an odd way life came to imitate art, for J L Toke inherited the house in 1866 but lost it after four hundred years of the Toke family living there.