Adam, previously senior director of A&R, will oversee the UK home of acts including Amy Macdonald, Razorlight and Metallica from Monday (July 19). He joined the label from Island five years ago, where he signed Busted and McFly.
Mercury Music Group president Jason Iley says Adam will "lead the label to the next level as a distinct repertoire stream within the Mercury group".
Nelson, formerly A&R executive, becomes director of A&R from Monday, with a brief to develop Mercury's domestic roster.
He joined Mercury from Parlophone, where he signed Lily Allen and Eliza Doolittle.
Iley says, "Jamie has tremendous drive and enthusiasm. He is an outstanding A&R executive and I am excited about what he can achieve in his new role."