The seventh-seeded Australians, who failed to convert three match points during the Wimbledon doubles final in July, needed three championship points to get the job done in New York after the Germans mounted a brief fightback in the second set.
"Grand Slam champion, that's got a great ring to it, especially after the heartbreak at Wimbledon," Thompson said during the trophy presentation.
"It got a little bit tense there at the end with match points. I thought 'I can't do it again' but we stuck with it and we got there."
With the win, Purcell and Thompson became the first all-Australian team to win a US Open men's doubles title since Todd Woodbridge and Mark Woodforde in 1996.
Purcell and Thompson struck first when they broke for a 3-2 lead and then enjoyed a pair of holds at love before serving out the opening set when they came back from 15-40 down.
In the second set, the Aussies again broke for a 3-2 edge but failed to convert two championship points when the German 10th seeds were serving in the ninth game and were then broken at love in the next game.
But Purcell and Thompson were not in a mood to let history repeat itself and pushed the frame to a tiebreak where they came back from 3-1 down before finally closing it out on their third championship point.
The win marked the second Grand Slam doubles title for Purcell, who triumphed in the 2022 Wimbledon final alongside compatriot Matthew Ebden, and the first for Thompson.
