Allsopp started his senior football career with Monbulk Rangers in 1994 as a 16-year-old, competing in the Victorian Provisional League Division 1. The following year he joined Croydon City in the Victorian State League Division 1 competition before joining then-NSL club South Melbourne in 1995. After two seasons with South, Allsopp made the move across town to Carlton S.C. for the 1997 NSL season.
Allsopp spent the 1998 Victorian Premier League season with Port Melbourne Sharks then moved to England to trial for Second Division club Manchester City, and after scoring in City's first friendly match against Newquay as well as some reserve team matches, he was signed for the club for the 1998–99 season. Allsopp's four goals in 25 games saw Manchester City promoted to the First Division, but he struggled to find a regular place in the team as the team in the 1999–2000 season, and was loaned out to Notts County.
He was loaned to Wrexham in early 2000, and scored four goals in just three league matches by end of the 1999–2000 season. The following season, he went on loan to Bristol Rovers, where he failed to score a goal in his four league appearances. Allsopp returned on loan to Notts County, and scored four goals in three matches before being bought for £300,000 by the club. In a three-season career, he scored 50 goals in 111 appearances (in all matches) before signing with Hull City for 2003–04. The move provided more success for Allsopp, as he scored 15 goals in his first season and seven in 2004–05 as a regular in the line-up.
Allsopp negotiated an early release from Hull, and decided to return to Australia to play for new club Melbourne Victory under Ernie Merrick, who had coached Allsopp during his time at the VIS. Despite making 20 starts in the 2005–06 season, Allsopp was not nearly as prolific as at his previous clubs, and only managed three goals for the year. The season featured Allsopp's 250th match in all league competitions.
The 2006–07 season saw a remarkable turnaround in Allsopp's scoring record at Melbourne. He finished the season as the league Golden Boot winner as highest scorer in the home and away fixtures. His tally of 11 made him the first A-League player to score a double figure tally and was one goal more than second place, teammate Archie Thompson. His 35 shots on target was also the equal highest with Newcastle Jets midfielder Nick Carle.
During the 2009–10 A-League season it was confirmed that Allsopp had signed with Qatari side, Al-Rayyan Sports Club, for an undisclosed fee. He had less than a year left on his contract with Victory.
On 22 September 2009, Allsopp made his debut for Al-Rayyan against Al-Kharatiyat, providing an assist for Amara Diane's goal.
Allsopp joined Major League Soccer club D.C. United on 18 January 2010. Allsopp and D.C. United mutually agreed to terminate his contract after just one season with the club. On 24 December 2010 Allsopp rejoined with his team mates at the Melbourne Victory, in a contract that will keep him there until the end of the 2012–13 season.
On 18 October 2012 he announced his retirement from professional football.
In 2013, Allsopp returned to play for his boyhood club, Croydon City Arrows. 2014 was a successful year for both Croydon & Allsopp, with the club winning the Victorian State League 4 East competition & promotion to State League 3 and Allsopp winning both the league Best & Fairest and Golden Boot Awards.
In 2013, Allsopp signed as a guest player for Launceston City FC. He featured in the round 15 National Premier Leagues Tasmania fixture between Launceston City FC and local rivals Northern Rangers FC. The match finished in a 6-0 win to Northern Rangers FC, with Allsopp picking up a yellow card for a reckless challenge.
In 2014 Allsopp again featured for the Tasmanian State League club as a guest player, in a round 5 National Premier Leagues Tasmania match against City's sister club Hobart Zebras FC. The game finished in a 4-2 win for City, with Allsopp scoring a hat-trick. This was a history making result for City and saw the end of a 25-game losing streak, with the club previously not winning a single league fixture since the new statewide league inception in 2013.
In 1995, he made his name in the under-17 World Championship, where he was tied top scorer with five goals, including one in Australia's 3–1 loss to eventual runners-up Brazil. His performance opened the door for his entrance into the Australian National Soccer League, signing with South Melbourne. He stayed with the club until 1997, having scored two goals in his 20 appearances.
Allsopp represented Australia again, this time at under-20 level in the 1997 FIFA World Youth Championship in Malaysia, with Australia making it through to the second round before being knocked out by Japan. On his return he was signed for Carlton SC, but was loaned out to VPL club Port Melbourne Sharks.
Meanwhile, Allsopp had become a regular member of Australia's under-23 national team (the "Olyroos") in 1999, and played three matches for one goal in 2000, but was not selected in the squad for the 2000 Summer Olympics.
Allsopp finally earned his first call-up to the Australia senior squad for a friendly match against Uruguay on June 2007 and came on as a 78th-minute substitute in the 2–1 defeat by the South Americans. On 23 May he got his second international appearance for Australia when he came on as a substitute for James Troisi in a friendly against Ghana.
National coach Pim Verbeek described Allsopp's performance against Indonesia in February 2009 as "absolutely hopeless", although he later added that "in the last games (for Melbourne) I have seen they were very good, so maybe it was because of Indonesia or the trip".
Allsopp was born in Melbourne, Victoria and is married with two sons.
(Correct as of 7 May 2010)
With Melbourne Victory: A-League Championship: 2006-2007, 2008-2009
A-League Premiership: 2006-2007, 2008-2009
Personal Honours:1995 FIFA U-17 World Championship Golden Shoe Winner
A-League Golden Boot: 2006-2007 with Melbourne Victory - 11 goals
Melbourne Victory Victory Medal: 2006-2007
Melbourne Victory Player's Player of the Year: 2006-2007, 2008-2009
D.C. United Top Scorer: 2010 - 9 goals