Wassall began his career in 1984 at Nottingham Forest, where he was a product of the youth system. He failed to win a regular first-team place, although he was a reliable stand-in to the likes of Des Walker, and was loaned out to Hereford United and Bury. Despite his limited opportunities, Wassall featured in two finals for Forest, winning a medal in the 3–2 victory over Southampton and playing in the 1–0 defeat in the 1992 Football League Cup Final against Manchester United. He left Forest for Derby County in 1992 in a £600,000 deal, and after an alleged dressing-room incident with manager Brian Clough. While at Derby, he had loan spells at Manchester City and Birmingham City. Wassall joined Birmingham full-time in 1997 in a £150,000 deal, but he struggled with injuries. After suffering an Achilles tendon injury, he left the Football League and played for Burton Albion. During his spell at Burton, Wassall scored in the 4–2 FA Cup defeat to Kidderminster Harriers in November 2000. He retired altogether in 2005.
Manager
After retiring as a player, in 2005, Wassall took charge of Burton's youth team. In 2009, Wassall became academy manager at Derby County under Nigel Clough. He has been credited with bringing through a number of players to Derby's first team, such as Mark O'Brien, Callum Ball, Jeff Hendrick, Mason Bennett and Will Hughes. In May 2013, Wassall was rewarded for his work, winning the President's Award at the club's awards night. In September 2013, Clough was sacked by Derby County, and his backroom staff were sent on gardening leave. Wassall remained at the club, continuing his work with the youth academy under the tenure of new head coach Steve McClaren. Clough had stated in March 2013: "The academy should almost be ring fenced and whoever comes in next, I hope they have the sense to leave it alone and let it continue working." After McClaren's successor Paul Clement was sacked in February 2016, Wassall was appointed head coach until the end of the season. He returned to his post of academy director on 27 May 2016, when Nigel Pearson signed a three-year contract to manage Derby County.
