On 28 November 2018, Berg Johnsen signed a professional contract with Viking FK.[2] He made his senior league debut for the club in a 2–1 win against Brann on 13 April 2019, coming on as a substitute.[3] On 29 May 2019, he was loaned out to 1. divisjon club Tromsdalen.[4] Berg Johnsen's contract with Viking expired at the end of the 2019 season.[5]
On 23 January 2020, he signed a two-year contract with 1. divisjon club Raufoss.[6] One year later, on 1 February 2021, he joined Danish Superliga club Randers.[7]
On 13 July 2023, Berg Johnsen signed a four-and-a-half-year contract with Allsvenskan club Malmö FF.[8][9]