Robertson was born on the Isle of Arran and left school at the age of thirteen to become a clerk and then a journalist. In 1878 he became a follower of secularist leader Charles Bradlaugh and became active in the secularist cause in Edinburgh, before moving to London to become assistant editor of Bradlaugh's paper National Reformer, subsequently taking over as editor on Bradlaugh's death in 1891. The National Reformer finally closed in 1893. Robertson was also an appointed lecturer for the freethinking South Place Ethical Society from 1899 until the 1920s.