Allen started his career with Oxford United, and made his debut under Brian Horton in the 1991–92Second Division campaign; he played fourteen games, helping the "U's" to avoid relegation. He made 31 appearances in 1992–93, as Oxford finished mid-table in the newly re-branded First Division. New manager Denis Smith played Allen in 45 games in 1993–94, though neither player or manager could prevent Oxford suffering relegation into the third tier. He played 36 games in 1994–95, as United missed out on the play-offs by six points. He played 24 games in the club's promotion winning 1995–96 campaign. Allen scored a total of 17 goals in 181 appearances in all competitions. Whilst at Oxford, he was capped twice for England at under-21 level, with both appearances coming in the 1995 edition of the Toulon Tournament, in 2–0 defeats to eventual finalists Brazil and France. He joined Frank Clark's Nottingham Forest on loan in February 1996, and made his Premier League debut on 2 March, in a 3–1 win over Sheffield Wednesday at Hillsborough. He scored his first top-flight goal a fortnight later, in a 1–1 draw with Middlesbrough at the Riverside Stadium, though would only make one further appearance in 1995–96. Allen moved to the City Ground permanently in the summer for a £400,000 fee. He then played 23 Premiership games in 1996–97, and also scored against Ipswich Town in the FA Cup; Forest were relegated at the end of the season. He appeared in just one league game under new manager Dave Bassett in Forest's 1997–98 promotion campaign, though he appeared in both legs of the 10–1 demolition of Doncaster Rovers in the League Cup, finding himself on the scoresheet for the 8–0 triumph at Belle Vue. Allen joined Lennie Lawrence's Luton Town on loan in November 1997, and played seventeen games, scoring once against Bristol Rovers at Kenilworth Road. He did not feature for Forest once his three-month loan spell at Luton ended. Allen did not feature under either Bassett or Ron Atkinson in 1998–99, and was instead loaned out to Cardiff City in October. He appeared five times for Frank Burrows's Third Division promotion winning side. In March, he was transferred to Brian Horton's Port Vale, playing five games, scoring once against Stockport County; the "Valiants" avoided relegation on goals scored. Allen signed with Andy Kilner's Stockport County in October 1999, and played sixteen games in 1999–2000. He then signed with Brighton & Hove Albion, but did not make a senior appearance. He spent summer 2001 on trial at Cambridge United, but was not offered a contract. He signed with Dover Athletic in October 2001, after manager Gary Bellamy said that he "destroyed the opposition" in the three games he played on trial. He played 17 Conference games in 2001–02, as the "Whites" finished bottom of the table and were relegated into the Southern League. He signed for North Leigh in 2006 and played with them until 2010. The "Yellows" were promoted out of the Hellenic Football League into the Southern League Division One South & West in 2007–08.
