Sealey joined Coventry City as an apprentice in 1976 and made his debut as a 19-year-old on 11 April 1977, in a 1-1 draw at Queens Park Rangers. He spent the next five seasons at the West Midlands club before joining Luton Town in 1983. He was a regular in the team for much of his time at Kenilworth Road, but he missed their 1988 League Cup triumph due to injury,his place being taken by Andy Dibble. A year later, Luton reached the final again and he was able to keep goal this time in a 3-1 defeat to Nottingham Forest. Sealey went on to lose his place in the line-up to Alec Chamberlain.
In December 1989, Sealey was loaned to Manchester United and made two league appearances during the final weeks of the season. He was named as goalkeeper for the 1990 FA Cup Final replay against Crystal Palace (even though United had signed him after the transfer deadline), after a poor display from Jim Leighton in the preceding 3-3 draw, and made several saves to help his side win 1-0. He later gave his winner's medal to Jim Leighton who had played throughout the cup run.
United signed Sealey on a permanent basis, and he was their regular goalkeeper throughout the 1990-91 season, keeping goal in their Cup Winners' Cup Final victory over Barcelona. He became a cult hero with United fans and got a standing ovation whenever he returned to Old Trafford. He was hoping to get a two-year contract, but was offered just a one-year deal and turned it down in favour of a transfer to Aston Villa. For much of the first half of 1991-92, Sealey was Villa's first-choice goalkeeper, but he then lost his place to long-serving Nigel Spink and never played for the club again.
In his second spell at Old Trafford, he made just two first-team appearances - once as a substitute when Schmeichel was sent off in the FA Cup Quarter Final against Charlton and the other in the League Cup final, which United lost 3-1 to his old club, Aston Villa. It meant his last four appearances for United were a Cup Winners Cup Final, two League Cup Finals and an FA Cup Quarter Final. At the end of the season he was given a free transfer and joined Blackpool, but within six months he had left Bloomfield Road and returned to the Premiership with West Ham.
Due to an injury crisis, Sealey made his Hammers debut as an outfield player, coming on as an attacking substitute against Arsenal. During his 18-month spell at the Boleyn Ground, he was understudy to Ludek Miklosko.
In December 1996, the 39-year-old Sealey returned to West Ham in an exchange deal for 47-year-old Peter Shilton. He made his last first-team appearance on the final day of the 1996-97 season, fittingly against Manchester United at Old Trafford. He had come on as a substitute for Ludek Miklosko, West Ham's regular first-choice goalkeeper.
At the end of the 1997-98 season he was loaned out to Bury but didn't make a first-team appearance. Upon his return to West Ham he was appointed as the club's goalkeeper coach, although he was still registered as a player during the 1999-00 season.
Sealey was still employed as West Ham's goalkeeper coach when he died of a heart attack on August 19 2001 at the age of 43. One of Sealey's pupils at West Ham, was current Derby County goalkeeper Stephen Bywater, who wears the number 43 on his shirt, as an homage to his former coach. Also at the club was his 18-year-old son Joe.
UEFA Cup Winners Cup winner 1991
League Cup finalist 1989, 1994