After turning professional, Liverpool-born Mountfield spent a season-and-a-half at local club Tranmere Rovers before moving across the River Mersey to play for Everton. He signed for £30,000 prior to the 1982–83 season. He formed a defensive partnership with Kevin Ratcliffe and helped the club to victory in the 1984 FA Cup Final against Watford and to the Football League title the following season. Everton also reached the FA Cup final that season, and won the European Cup Winners Cup in Rotterdam. He also won another League championship medal in 1987. In league matches, he scored an average of one goal every 5.6 matches and in one season, he scored 14 goals in all competitions.