Mäenpää made his international debut for Finland in June 2008, at the age of 23, and has since had over 20 caps, including appearing in 2014 FIFA World Cup qualification.

During summer 2006, Mäenpää joined Dutch Eerste Divisie side FC Den Bosch on a free transfer.

In summer 2009, Mäenpää joined Dutch Eredivisie side Willem II on a free transfer, making 18 league appearances.

[15] Mäenpää signed a short-term contract with Stoke City on 27 November 2020 to act as cover for Josef Bursik following injuries to Adam Davies and Angus Gunn.

[19] On 14 May 2022, in a Serie A match against AS Roma at Stadio Olimpico, Mäenpää had a man-of-the-match performance with a record-number of 16 saved shots (8 of which came from inside the box).

[20] On 4 August 2023, after playing professional football abroad for 20 years, Mäenpää returned to Finland and signed with his former club HJK Helsinki on a deal for the rest of the 2023 season.

[21] A couple of weeks later, Mäenpää had made his debuts in the Europa League and in the Finnish premier division Veikkausliiga, at the age of 38.

Mäenpää made his debut for the Finland national team on 2 June 2008 in a friendly match in Veritas Stadion, Turku against Belarus when Stuart Baxter chose him for the starting line up.

[27] He played his first FIFA World Cup qualification game on 12 October 2012 in Helsinki Olympic Stadium when Mixu Paatelainen chose him for the starting eleven in a match against Georgia.