In all, he played 15 games and scored 7 goals for the Danish national team from 1908 to 1920. The three times Middelboe played for his country while at Chelsea, he always represented KB, as opposed to Chelsea. Nils Middelboe is renowned as the first non-British international to play for Chelsea, during his stint at the club between 1913 and 1923. Originally he had signed for Newcastle United; however, with the permission of the Newcastle Director, he switched to Chelsea. He made his debut on 15 November 1913 against Derby County at Stamford Bridge - a 2–1 win for Chelsea. As a show of faith from the players, he was appointed captain, despite never having previously played for the Chelsea first team. In all, he played 46 matches for Chelsea until 1922. After his days at Chelsea, he played for the amateur sides of Corinthian F.C. and Casuals F.C. until 1926. In 1929 he was appointed a Director of the English League Division Three south East London based side Clapton Orient. He returned to Denmark in 1936. It was through Middelboe's contacts in English football, that English coach Edward Magner was hired to coach the Danish national team in 1939. As an interpreter, Middelboe mediated Magner's tactical instructions on the modern WM formation to the team, a formation Middelboe himself was a known critic of. As a coach for KB, he led the team to the Danish football championship in 1940. 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.