Niemi was born in Oulu, Finland and started football in local club Oulun Luistinseura (OLS) and Rauman Pallo before joining HJK Helsinki in 1991.

Ahead of the 1992 season, Niemi went on a trial with Belgian side Club Brugge but he did not earn a contract and stayed at HJK Helsinki.

[17] After resuming his first choice goalkeeper role following his return from injury, Niemi helped HJK Helsinki go on an eight match unbeaten streak between 11 July 1992 and 8 August 1992.

[21] In the 1993 season, Niemi continued to retain his first-choice goalkeeper role at HJK Helsinki and the club made a good start at first, but the their form soon declined that saw them out of being a title contenders.

[34] Despite HJK Helsinki losing remaining three matches of the 1994 season, the club finished third place in the league that saw them qualify for the UEFA Intertoto Cup.

[35] His season ended in a high note when he helped HJK Helsinki win the Finnish League Cup after beating FC Jazz 2–0.

[46] Niemi made his debut for the club, starting the match, in a 5–0 win against Gøtu Ítróttarfelag in the first leg of the UEFA Champions League's first round.

[63] A week later, Niemi started the Scottish League Cup final and was able to make up his amends when he helped the club beat St Johnstone 2–1 to win the tournament.

[73] Since joining the club, he quickly established himself as Hearts’ first choice goalkeeper, successfully dispatching from Gilles Rousset and Roddy McKenzie.

[82] Despite losing 1–0 against his former club, Rangers on 17 September 2000, newspaper The Herald said about his performance: "he found himself playing spectator for much of the 90 minutes as the opposition sat back, not through choice, but because they had no alternative amid an incessant attack ".

[83] By mid–November, Niemi conceded fourteen goals in the last three matches, including one in a 5–2 loss against Celtic in the quarter-finals of the Scottish League Cup, but he remained the club's first choice goalkeeper.

Ahead of the 2001–02 season, Niemi was linked with a move back to re-joining newly promoted Premiership side Charlton Athletic, but he ended up staying at Hearts.

[111] Niemi made his debut for Southampton against his former club, Charlton Athletic on 21 September 2002 and kept a clean sheet, including saving a 30-yard free-kick from Claus Jensen and a 25-yard shot from Chris Bart-Williams, in a 0–0 draw.

Ahead of the 2003–04 season, Niemi was linked with a move to Premiership's champions, Manchester United, but he ended up staying at Southampton, due to his calf injury.

[127] On 4 October 2003, he made his first appearance of the season against Newcastle United and was faultless from conceding a 43rd-minute goal from Alan Shearer, as the club lost 1–0.

[128] His second appearance of the season came in the second leg of the UEFA Cup first round against FC Steaua București and conceded an 83rd-minute goal from Claudiu Răducanu, as Southampton lost 1–0 and was eliminated from the tournament.

[138] In a match against Blackburn Rovers on 28 February 2004, he produced good performances and "thwarted striker Andy Cole on a number of occasions and his array of saves" before conceding a goal from him, as the club drew 1–1.

Local newspaper, Southern Daily Echo, praised Niemi's performance, saying: "It's a difficult quality to explain or quantify, but like many great players, the Finn has a real presence.

[143] Ahead of the 2004–05 season, there was much speculation regarding Niemi's future at Southampton with a host of leading Premier League clubs rumoured to be interested in his services, such as Manchester United and Arsenal.

[155] With Premier League clubs continuously remain interested in signing him in the January transfer window, he ruled out leaving Southampton.

[161] In the last game of the season, Niemi was unable to help Southampton avoid relegation despite his best effort after losing 2–1 against Manchester United.

[188] Since returning from injury, he remained the club's first choice keeper and found himself in a number of defeats that pushed Fulham into the relegation places.

On 7 August, Niemi confirmed he had signed for Portsmouth on a one-year deal as backup to David James and to work with the goalkeeping coaches.

It wasn't until on 27 January 1994 when Niemi made his first appearance for Finland's senior team in almost a year, and kept a clean sheet, in a 0–0 draw against Qatar.

[215] In mid–1999, Niemi lost his place as the national side's first choice goalkeeper role, with Jani Viander and Peter Enckelman preferred for the rest of the year.

[219] Despite helping Finland five matches unbeaten for the rest of the FIFA World Cup qualification, the national side failed to advance to the playoffs.

[223] At the start of the FIFA World Cup qualification, he helped the national side go on a three match winning streak, including keeping two clean sheets.

[224] On 13 October 2004, Niemi started in a match against the Netherlands and conceded three goals, as Finland lost 3–1, in what turned out to be his last appearance for the national side.

[3] On 27 June 2014, Niemi was appointed goalkeeper coach at Championship side Brighton & Hove Albion by former Finland teammate Sami Hyypiä.

[238][193] Following his end of his professional football career, Niemi returned to Finland to focus on his family while balancing his role as a goalkeeper coach.