Mercer was promoted from the academy in the 2009–2010 season. He was named man-of-the-match in Bath's Aviva Premiership loss to Wasps in 2010 and was named club captain in 2016.
Ospreys
In August 2017, Mercer joined Welsh Pro14 side Ospreys on a season-long loan deal, joining in time for the opening round of the 2017–18 Pro14 season.[1]