At the age of 20 he scored his first national team goal against Norway in the quarterfinals at his second game of the tournament.

19 February 2013, Lajunen was traded by the Predators to the St. Louis Blues for fellow minor leaguer Scott Ford.

[2] On 5 May 2017 Lajunen agreed to a two-year contract with HC Lugano of the National League (NL).

[3] On December 24, 2018, Lajunen was signed to an early two-year contract extension by Lugano through to the end of the 2020–21 season.

On 20 May 2021, Lajunen left the Swiss National League as a free agent after four seasons and returned to the SHL, in agreeing to a two-year contract with Örebro HK.