[2] Myllymaa started playing ice hockey in Porin Ässät when he was five years old.
Myllymaa played his professional Liiga debut with Porin Ässät on 18 September 2021, against Oulun Kärpät.
Myllymaa scored the first goal of his Liiga career in his fourth match on 28 January 2022 against SaiPa.
On 9 October 2023, Myllymaa signed a contract with the Austrian side EC Red Bull Salzburg of the ICE Hockey League.
Myllymaa was put on the fourth line for the first squad in a Champions Hockey League (CHL) matchup against the Finnish team Lahti Pelicans.