The first Indianapolis 500 was held in 1911.
The inaugural International 500-Mile Sweepstakes Race took place at Indianapolis Motor Speedway on May 30, 1911. It was run on Decoration Day, the holiday later officially renamed Memorial Day, and attracted a field of 40 starters. Ray Harroun won for the Marmon Motor Car Company in a Marmon-based Wasp.
The speedway itself had opened in 1909 and hosted several smaller races before its owners decided to concentrate on a single major long-distance event. Founder Carl G. Fisher settled on a 500-mile race, believing the distance would provide a compelling spectacle and could be completed before darkness.
A common mix-up is treating 1909 as the answer because that is when the track opened and began hosting motorsport. The Indianapolis 500, however, did not exist until the inaugural race in 1911. The event was not held during the World Wars, so its running number and calendar year are not always identical.