He represented the Denmark national football team at the 1908, 1912 and 1920 Summer Olympics, winning silver medals in 1908 and 1912.

From 1904 to 1913 Nils won five KBU football championships with KB, while he also excelled in athletics competitions.

Occupying the half-back role, he won silver medals with Denmark in both the 1908 and 1912 Olympic Games, the latter as captain of the side.

Nils Middelboe is renowned as the first non-British international to play for Chelsea, during his stint at the club between 1913 and 1923.

In 1929 he was appointed a Director of the English League Division Three south East London based side Clapton Orient.

In April 1946, Chelsea played KB in a friendly, arranged to celebrate that club's 70th anniversary.

The chairman of KB announced that, owing to Middelboe's "long and happy association" with Chelsea, there was no other club they would rather be facing.