He previously served as head coach of the Finnish national ice hockey team, and of the Espoo Blues of the Liiga and Jokerit of the KHL.

Marjamäki took over the reins at fellow Liiga club Oulun Kärpät prior to the 2013-14 season and coached the team to back-to-back Finnish championships in 2014 and 2015.

In August 2017, Marjamäki announced that he would not take the +2 year option available in his contract, despite the Finnish Ice Hockey Association being open to the extension, and would leave the position at the conclusion of the 2017–18 season.

At the men's ice hockey tournament of the 2018 Winter Olympics, a loss to Canada in the quarterfinals prevented the Finns from progressing to fourth position or higher, ultimately placing sixth.

He is married to Finnish Olympic ice hockey player, Sari Marjamäki They have two children, a daughter (born in 2008) and a son.