Absolutely! The first rearview mirror was used on a race car in the very first Indianapolis 500 in 1911 - on the Ray Harroun-driven Marmon Wasp.
From www.popularmechanics.com:
Driver Ray Harroun stirred controversy at 1911's inaugural 500 by ­entering the only single-seater in the field, a stinger-tailed Marmon Wasp. His fellow drivers, each of whom had a "mechanician" riding shotgun, knew that by motoring solo, Harroun would have an edge in weight and aerodynamics. They complained that without a spotter beside him, Harroun would be blind to racers closing in from behind and thus be a danger to all. His solution: a rectangular mirror mounted to the cowl on four steel dowels. Harroun won the race, and by the mid-teens the device—marketed as a "mirrorscope" or "cop-spotter"—was a popular aftermarket accessory. Incidentally, Harroun's mirror was a bust: It vibrated so fiercely during the race that he couldn't see a thing in it.
Read more:
15 Ways the Indy 500 Changed How You Drive - Indianapolis 500 100th Anniversary - Popular Mechanics