Mercer, a wing-half, was a regular in the Preston team for the next five seasons. However, despite the goals of Tommy Roberts, it was a struggling team.
Preston North End did very well in the FA Cup in 1921. Led by veteran defender, Joseph McCall, the club defeated Newcastle United (3-1), Barnsley (3-0), Arsenal (2-1) and Tottenham Hotspur (2-1) on the way to the final against Huddersfield Town. Preston lost to the only goal of the game, a penalty conceded by Tommy Hamilton. It was awarded after Hamilton tripped Huddersfield's outside-left Billy Smith. Hamilton admitted the offence but claimed it was outside the penalty area.
During Mercer's time at the club, Preston finished 19th (1919-20), 16th (1920-21), 16th (1921-22), 16th (1922-23), 18th (1923-24) and 21st (1924-25).
After they were relegated in 1925, Mercer joined Blackpool. However, he did not stay long at his new club and moved on to non-league football at Lancaster Town. Later he played for Boston Town.
William Mercer died in 1975.