Following an impressive three goals in six matches at the 1972 Summer Olympics, Simonsen moved to Germany to play professionally for defending German Bundesliga champions Borussia Mönchengladbach.

[2] In the final, Simonsen scored a memorable goal from a powerful shot to level the game at 1–1, but Mönchengladbach eventually lost 3–1.

The win was notable in that Simonsen's native Denmark was not among the top footballing nations in the 1970s, leaving little room for him to impress at the international tournaments.

Instead, Simonsen waited for his contract to expire and moved to FC Barcelona in 1979, rejecting offers from Hamburger SV, Juventus, and several Arabian clubs.

Simonsen's ten goals saw him as third top goalscorer behind new players Quini (20) and Bernd Schuster (11),[10] as Barcelona finished in fifth place in the 1980–81 La Liga.

In the 2–1 final victory against Standard Liège,[12] Simonsen scored the deciding goal on a header to help Barcelona lift the trophy.

[13] He rejected offers from Real Madrid and Tottenham Hotspur, in order to play for a club with less stress and attention.

[8] Despite scoring nine times in 16 appearances, the club had trouble funding his transfer and wages after three months, and he was put up for sale.

[6] Simonsen debuted for the Danish national team under manager Rudi Strittich in the July 1972 friendly match against Iceland.

[4] He played a crucial part for the Danish national team under manager Sepp Piontek, in Denmark's qualifying campaign for the 1984 European Championship.

Simonsen scored one of the most important Danish goals ever in this game, as he converted a penalty kick against English goalkeeper Peter Shilton.

[16] The 1984 European Championship main tournament was a short experience for Simonsen, as he broke his leg in a challenge by Yvon Le Roux in Denmark's first match against France.

He only played a single match at the tournament, coming on as a substitute against West Germany, as younger players had surpassed him.

[4] However, some sources chose to include Simonsen's appearance in a February 1981 charity match, to tally his national team career as 21 goals in 56 games.