Cox was born in London, England, the youngest of five children. As a child, he attended Ashdown House and Sherborne School. He trained at the Bristol Old Vic Theatre School.
His most recent role was that of Tristan Thorn in the 2007 movie Stardust. Cox has reportedly been close friends with Claire Danes ever since principal photography of the film Stardust.
He will next be seen in the forthcoming film Stone of Destiny in which he plays Ian Hamilton. In 2008 he will make his West End debut in a a double bill of Harold Pinter works at the Comedy Theatre