Born in Carlisle, Cumbria, Jansen's career started at Carlisle United, where he scored 10 league goals. He turned down the chance to join Manchester United before signing for Crystal Palace in 1998 for £1 million, where his impressive displays earned him a move to Premier League side Blackburn Rovers for £4.1 million.

Jansen showed signs of things to come when he scored against Tottenham Hotspur on his debut in January 1999.

Blackburn were relegated to the First Division in Jansen's first season, but he was the star of the team which won promotion back to the Premier League in the 2000/01 season, finishing as the league's second top scorer, after Fulham's Louis Saha, with 23 goals.

He continued to impress in the following season, scoring the first goal in Blackburn's 2-1 League Cup final victory against Tottenham Hotspur in 2002.

His good form led to a fully justified England call-up for the friendly game against Paraguay on 17 April 2002; however, a stomach illness forced him to miss the game and consequently denied him the chance to claim a place in the World Cup 2002 squad. He previously had played 6 times for the under-21s.

A motorcycle accident in Rome, during the summer of 2002, nearly killed Jansen and long-term hospitalisation followed (he spent four days in a coma).

In the seasons after the accident he struggled to regain his place in the Blackburn side which led to him being loaned to Coventry City in 2003, where he scored 2 goals in 9 appearances.

He made another attempt to force his way into the Blackburn team at the start of the 2004/05 season, scoring 2 goals in 8 appearances.

However, it was felt that he had still not recovered mentally from the accident and was struggling to cope with the pressures of Premier League football. As a result, Blackburn decided it best for him take another break from football and sent him to visit a sports psychiatrist in America.

In May 2005, Jansen signed a 2 year 'pay as you play' contract with Blackburn, suggesting that the club was ready to give him one final chance to establish himself in the team after his injury, and he returned to action in a pre-season friendly against Morecambe in July 2005.

However, Jansen once again struggled to gain a first-team place once the season started. Blackburn agreed to release him from his contract on 11 January 2006, two days later he signed for Bolton Wanderers on a six month contract, after interest from Stuart Pearce's Manchester City failed.

His first game for his new club came, naturally, at Ewood Park. However, his stay at the Reebok Stadium was not a success and he was released in May of the same year.

In the summer of 2006, Jansen had a trial with the New York Red Bulls but the move led to nothing permanent due to rival team Columbus Crew allegedly exercising a loophole in transfer rules to block any move.

Bolton manager Sam Allardyce invited the player back to England and he continued to train with Bolton. Then iIn the summer of 2007 he returned to his original club, Carlisle United, to train in an attempt to regain his fitness.

In November 2008, Blackburn announced that Jansen would return in a casual agreement to help regain fitness in a bid to have one last go at reclaiming the promising and glittering career he once had in professional football.

Having trained with the club for 2 months he left Brockhall with the arrival of new manager Sam Allardyce.

On February 18, he joined Football League One side Huddersfield Town on trial and was given a run-out in their reserve match against Rotherham United the same day, in which he scored the opening goal of a 2-2 draw.

On March 11, he agreed a deal with Blue Square Premier side Wrexham until the end of the season.