Johnsen started his football career in Vålerenga and Tønsberg, staying there for two seasons. After leaving Tønsberg he went to Andalusia, to the CD Úbeda Viva, where he became the first foreigner in the history of the club. Johnsen spent close to three months with the club, where he played four games and scored a goal before signing fourth tier Spanish team Antequera in January 2012.[4] Eight months later, on 5 August 2012, Johnsen joined Segunda División B side Atlético Baleares,[4] where he played alongside Walter Pandiani.[5]
Atlético CP
On 17 July 2014, Johnsen signed with Portuguese club Atlético CP on a two-year deal.[6] In August, he won the award for Player of the Month in the Segunda Liga, after scoring nine goals in five games.[7]
His strong performances saw him being scouted by such teams as Benfica, Sporting CP and Belenenses, but the transfer never arrived.[8][9] According to the Portuguese press Benfica offered €300,000 plus two of their players, but Atletico wanted €500,000 and thus rejected the offer.[10]
Litex Lovech
After half a season, 31 matches and 16 goals with Atletico, Johnsen joined the Bulgarian team Litex Lovech for two and a half years.[11] He made his debut on 28 February 2015 in a home win against Ludogorets Razgrad.[12] On 23 May he scored a hat trick in a match against Ludogorets for a 4–2 win.[13] On 12 December 2015, he was sent off after an altercation with Miguel Bedoya in an ill-tempered A PFG match against Levski Sofia that was eventually abandoned after the Litex players were ordered off the pitch.[14]
Johnsen moved to ADO Den Haag for an undisclosed fee on 31 July 2017.[16] In his first season he scored 19 goals in 34 official matches.
AZ Alkmaar
After a great season at ADO Den Haag, Johnsen transferred to AZ Alkmaar in 2018.[17] His performance was substandard leading to a half-year loan spell at Rosenborg BK.[18]
On 22 July 2023, Johnsen signed with K League team FC Seoul.[22]
Torpedo Kutaisi
On 24 January 2024, Johnsen joined Georgian club Torpedo Kutaisi on a 1+1 deal.[23] He opened his goalscoring account with a brace in a 4–1 win over Samtredia.[24]
International career
Due to his dual nationality, Johnsen was able to play for both Norway and the United States.[25] Johnsen made his Norway debut in June 2017.[26]
^"Nordmenn i spansk fotball" [Norwegians in Spanish football]. Fotballsonen (in Norwegian). 5 April 2013. Archived from the original on 24 September 2015. Retrieved 25 June 2015.