Johnsen started his football career in Vålerenga and Tønsberg, staying there for two seasons, until he signed with fourth tier Spanish team CD Úbeda Viva, in Antequera in January 2012. Eight months later, on August 5, 2012, Johnsen joined Segunda División B side Atlético Baleares, where he played alongside Walter Pandiani.
On July 17, 2014, Johnsen signed with Portuguese club Atlético CP on a two-year deal. In August, he won the award for Player of the Month in the Segunda Liga, after scoring nine goals in five games. His strong performances saw him being scouted by such teams as Benfica, Sporting CP and Belenenses, but the transfer never arrived. According to the Portuguese press Benfica offered €300,000 plus two of their players, but Atletico wanted €500,000 and thus rejected the offer.
After half a season, 31 matches and 16 goals with Atletico, Johnsen joined the Bulgarian team Litex Lovech for two and a half years. He made his debut on February 28, 2015 in a home win against Ludogorets Razgrad. On 23 May he scored a hat trick in a match against Ludogorets for a 4–2 win. On December 12, 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.
On July 22, 2016, Johnsen completed a move to Scottish Premiership side Heart of Midlothian, on a three-year deal. He made his debut on August 20, 2016, appearing as a substitute in a 5–1 victory over Inverness Caledonian Thistle played at Tynecastle, where he assisted a Sam Nicholson goal with one of his first touches.
