He started his senior career at Lens Reserves in 2003. During winter 2006 transfer window, he joined Telstar on loan until the end of the season. He did not feature in any Ligue 1 match in the first team of RC Lens. His contract with Lens expired on 30 June 2006.
During summer 2006, he joined Dutch Eerste Divisie side FC Den Bosch on a free transfer. He made his breakthrough in season2006–07 when he played 27 league matches. He made a total of 67 league appearances during 3 seasons. His contract with FC Den Bosch expired in summer 2009.
In summer 2009, he joined Dutch Eredivisie side Willem II on a free transfer, making 18 league appearances. He left Willem II in summer 2011, by terminating his contract after they got relegated to Eerste Divisie.
On 3 October 2011, he joined AZ Alkmaar on a free transfer on a contract until the end of the season, joining compatriot and team captainNiklas Moisander. However at AZ, he found himself demoted to third-choice, behind Esteban Alvarado and the veteran Erik Heijblok. He left the club after only one season, without making a single appearance for the AZ's first team.
On 29 May 2012 it was announced that Mäenpää had signed a three-year contract with fellow Eredivisie side VVV-Venlo. Despite being sent off 3 minutes into his debut match, the fans voted him as best Venlo player of the season 2013-14. In the summer of 2015 Niki Mäenpää left VVV Venlo on a free transfer after his contract expired
Brighton & Hove Albion
On 6 July 2015 it was announced that Mäenpää had signed a contract with English side Brighton & Hove Albion following his release from VVV-Venlo. He made his Brighton & Hove Albion league debut in the 3–0 victory over Rotherham at Falmer Stadium on 16 August 2016. At the end of 2017-18 season, he left the club.
Bristol City
On 1 August 2018, Mäenpää signed for EFL Championship club Bristol City. On 4 August 2018, Mäenpää made his debut in a match against Nottingham Forest. On 21 May 2019, it was announced an option to extend his contract was triggered and he would be remaining with The Robins for the 2019–20 season.
International career
Mäenpää was 18 years old on 1 April 2003 when he debuted in Finland's youth team in a match against Sweden in Malmö. He played his first game in Finland national under-21 football team 30 March 2004 in Corradino in a match against Malta. He gained 13 caps in Finland U21-team. He made his debut for the Finnish national team on 2 June 2008 in a friendly match in Veritas Stadion, Turku against Belarus when Stuart Baxter chose him to the starting line up. During 2012 he established himself as a regular in the Finland national team and gained 6 caps. He played his first FIFA World Cup qualification game on 12 October 2012 in Helsinki Olympic Stadium when Mixu Paatelainen chose him to starting eleven in a match against Georgia. He played one of his best games in March 2013 when Finland got a surprising 1–1 away draw against Spain in the 2014 World Cup qualifying. He established himself as the national team's regular goal keeper and played 7 out of 8 matches in the qualifications and all together 20 matches during years 2012-2014.
Career statistics
Club
Notes:
Play Off Promotion - Relegation Matches 2007 & 2008 = For promotion 2010 = Relegation battle 2013 = Relegation battle 2014 & 2015 = For promotion
