[1] Johannessen played for the national team and represented Norway at the 2012 Eisenhower Trophy with Anders Engell and Kristoffer Ventura, where they finished sixth.

He played in the European Boys' Team Championship three times and won silver in 2010, after he was beaten by Thomas Pieters of Belgium by 3 to 2 in the final.

[2] He had his international breakthrough at 19 during the qualification for the 2014 Open Championship, where he lowered the course record at Bruntsfield Links near Edinburgh by four strokes, from 67 to 63 (eight under par).

[4] Johannessen joined the 2015 Nordic Golf League, where he secured his first professional victory at the 2015 Holtsmark Open on home soil, following a playoff with Petter Bocian of Sweden.

In June 2021 he qualified for the 2020 Summer Olympics in Tokyo together with Viktor Hovland, just ahead of Kristoffer Ventura, thanks to the runner-up finish at the Czech Challenge which earned him 6 OWGR points and a career-high world rank of 289.