In his 1948 Fine Art of Mixing Drinks, David A. Embury says of the Sidecar's origin: "It was invented by a friend of mine at a bar in Paris during World War I and was named after the motorcycle sidecar in which the good captain customarily was driven to and from the little bistro where the drink was born and christened." Embury doesn't name the bar but it's commonly assumed that he meant Harry's New York Bar and that the cocktail was created by its owner, Harry MacElhone.
However, in early editions (1919 and 1922) of Harry's own ABC of Cocktails he credits the drink to Pat MacGarry "the Popular bar-tender at Buck's Club, London", but in later editions appears to take credit for the drink himself.
In his 1922 Cocktails How To Mix Them, Robert Vermeire writes, "This cocktail is very popular in France. It was first introduced in London by McGarry, the celebrated bar-tender of Buck's Club." - Difford’s Guide