Friday's (Saturday AEST) win gave Siniakova her 12th grand slam doubles title and made her the first player to complete the career Grand Slam with two different players, having previously done it with fellow Czech Barbora Krejcikova.
It also made Townsend the sixth active women's player to complete the career Grand Slam in doubles.
"To be able to come here and win as an adult, and be able to play on Arthur Ashe and to hold this trophy, it's something that's so special to me because I don't forget the things that I've gone through," Townsend told reporters.
After a sluggish start, Siniakova and Townsend came alive in the second set where the 2025 finalists fell behind 40-30 in the opening game before reeling off 23 of the next 27 points to force a decider.
The 2025 finalists then consolidated an early break with a Siniakova hold at love to move ahead 3-1 before Townsend served up a four-point game of her own for a 4-2 lead that set them on their way.
In the opening frame, the top-seeded pairing reeled off 13 consecutive points en route to reaching 4-4 before Siniakova, serving with the sun in her face and looking to force a tiebreak, double-faulted three times, including on set point.
"There was a lot of nerves, a lot finals but in the end we managed to do it," Siniakova said during the trophy presentation.
"I am just so proud of our fight, of our relationship, so thank you for staying by my side and I hope it's not the end and we will come and play more finals."
Siniakova and Townsend won the Wimbledon women's doubles title in 2024, the Australian Open in 2025 and this year's French Open.
