Rarely do we have proof of a sandwich's invention. It's not as if the creators draw up documentation and have it notarized; usually they're just hungry. So the Reuben, as with many foods, has several origin stories. I like the one about Reuben Kulakofsky, an Omaha grocer said to have made up the sandwich in 1925.
In Nebraska, home of sandwich precursor the bierock, beef and cabbage were a familiar combination, and according to lore, the corned beef/Swiss cheese/sauerkraut on rye combo was first conceived at the Blackstone Hotel in Omaha.
Like Earl of Sandwich John Montagu, who is credited with inventing the first sandwich during a long card game, Kulakofsky (often called Reuben Kay) needed to nosh while playing cards and supposedly came up with the Reuben to feed his late-night poker buddies. Like the hot brown sandwich, you might call the Reuben a pioneering drunk food.
The hotel's owner, Charles Schimmel, added the sammie to the menu and named it after Reuben; in 1956, the sandwich won a national competition, providing the first documentation of the name for the Oxford English Dictionary. A Nebraska newspaper columnist dug up menus from the 1930s and '40s that featured Reubens, though Mr. Kulakofsky's obituary made no mention of his claim to fame.
I actually have a friend of a friend whose grandmother's great uncle was Reuben Kulakofsky — four degrees of separation — but he didn't hear stories about the Reuben at his grandmother's knee or anything. Maybe Reuben was just too humble, because it's another not-so-humble inventor who typically gets credit for the Reuben. Stay tuned.