To succeed at something after being doubted or scorned is to have the last laugh. Having the last laugh is a kind of vindication — you have to go through some unpleasantness first, often being ridiculed or considered a failure, before ultimately triumphing. Many movie plots rely on the satisfaction of a character getting the last laugh, like a scientist whose belief in aliens is mocked until the day a UFO lands in her hometown, or an unpopular kid at school who has the last laugh after being voted class president.

Source: Vocabulary