The French pair beat Henri Kontinen of Finland and John Peers of Australia 6-4, 7-6 (1) in Sunday's final.
Herbert and Mahut now own four major doubles titles.
They won the U.S. Open in 2015, Wimbledon in 2016 and the French Open in 2018.
They are the eighth men's doubles team in tennis history to win each Grand Slam tournament at least once.
Herbert and Mahut saved all four break points they faced Sunday, all while Herbert served in the fourth game of the second set.
The match's only break came when Kontinen served at 4-all in the first set.