Already in 1950 in the Canadian National Exhibition, the visitors lost (it seems) most of their games of tic-tac-toe against Bertie the Brain, which was a custom computer: https://en.wikipedia.org/wiki/Bertie_the_Brain#Gameplay

Excellent! Upon further digging, I found out that a chess grandmaster, one Savielly Tartakower, was defeated by El Ajedrecista [1], a machine that could play chess endgames. This was 1951, so before the Los Alamos lab assistant, but a bit after the defeat of customers by Bertie the Brain.

The Ajedrecista had been around since 1912, so I would expect somebody must have been defeated by it in the years before 1951.

[1]. https://www.chessprogramming.org/El_Ajedrecista