[1] He was one of the listed players of the Danish football squad that competed in the 1908 Summer Olympics, but did not play in any of the matches at the competition.

When Einar was 8 years old in 1891, the Middelboe family moved from Brunnby in Scania to Frederiksberg in Copenhagen,[4] and on 26 February 1910 he became a Danish citizen.

[3] He ran it as sole owner until 1941, when his son, Steen Middelboe, and four employees, Vilhelm Scherling, Harry Clausen, Leo Bursøe, and Helmuth Jørgensen were admitted as co-owners.

[citation needed] After his football career, Middelboe became a member of KB's board in 1914, and in 1931 he became the club's deputy chairman.

[3] Despite the fact that he was still a Swedish citizen at the time, Einar Middelboe was selected for the Danish squad that won a silver medal football tournament of the 1908 Olympics in London.