Mercer struck after 36 minutes, selling the Salford defence an outrageous dummy before crashing over from 20 metres.
That try was converted by Graham Holroyd and they went on to produce a strong display after the interval. Carl Gillespie and Daryl Cardiss crossed in a five-minute spell in the second half, with the influential Martin Moana setting up both scores, and the Reds could not come back.
The result leaves Salford in trouble, three points above the bottom club, Hull.
On Saturday, London were denied the chance to avenge their Challenge Cup final defeat as Leeds beat them 22-16 and moved into second place in Super League.
The Broncos were made to pay for poor discipline in crucial field positions, with Leeds clinching victory thanks to the boot of Iestyn Harris who kicked five goals, including four penalties.Reuse content