Yes, it was and one of the movies it beat out was 'Random Harvest', which, had the same three screenplay writers as 'Mrs. Miniver'. The movie also beat out 'Pride of the Yankees'. My all-time favorite actor, James Cagney, won best actor that year for 'Yankee Doodle Dandy'.
Coincidentally there is a nice picture of Greer Garson on page 34 of today’s Life magazine. I think she is endorsing silverware or something, but I prefer looking at her.