Who was the prime minister when war was declared in 1939?

Stanley Baldwin. Ooops, take me out & give me a kicking ! It's the national Government of Ramsay MacDonald.
Ramsey MacDonald- he had been in office since late August 1931 as head of a National Labour government, and remained in power until 7th June '35, when he was replaced by the Conservative Stanley Baldwin.

Both Baldwin and MacDonald had held the post before- MacDonald for just under a year from January - December 1924, and Baldwin from '24 to '29.