Campion was born in London, the son of John Campion, a clerk of the Court of Chancery, and Lucy (née Searle – daughter of Laurence Searle, one of the Queen's serjeants-at-arms). Upon the death of Campion's father in 1576, his mother married Augustine Steward, dying soon afterwards. His stepfather assumed charge of the boy and sent him, in 1581, to study at Peterhouse, Cambridge as a "gentleman pensioner"; he left the university after four years without taking a degree. He later entered Gray's Inn to study law in 1586. However, he left in 1595 without having been called to the bar.