After the one-year loan, 1. FSV Mainz 05 exercised the option.
Ingvartsen moved to Köpenick in the first Bundesliga season of 1. FC Union Berlin. The striker played 60 games for "the Eisernen" and contributed eight goals. In addition to keeping the club in the Bundesliga, Ingvartsen also helped to qualify for the UEFA Europa Conference League in his second year with Union.