Mercer was born at Cantril Farm in 1969 and joined local club Liverpool as a trainee and turned professional at the age of 18.

He impressed during his time coaching at Ashton Gate and was subsequently brought to Hillsborough in the summer of 2004 by Chris Turner shortly before his sacking as Sheffield Wednesday manager.

[1] Following the sacking of Paul Sturrock on 19 October 2006 Mercer acted as caretaker assistant manager to Sean McAuley before the arrival of Brian Laws and Russ Wilcox on 6 November 2006.

[2] Following Brian Laws' appointment as manager of Premier League side Burnley in January 2010, Mercer joined the club as goalkeeping coach.

On 10 July 2023, Billy was signed to a short term deal with Everton to fill-in for goalkeeping coach Alan Kelly who was sidelined due to knee surgery.