Gauff, 15, needed only an hour and five minutes to beat Japan’s Hiroko Kuwata in a 6-1 6-2 win.
It will be her third WTA main draw appearance after Miami and Wimbledon.
After her SW19 exploits, where she beat Venus Williams on her way to the last 16, Gauff’s qualifier was watched by a near sell-out crowd in the US capital.
And she received a standing ovation after clinching victory with a wonderful cross-court winner against 28-year-old Kuwata.
Now ranked 143rd in the world after her success at the All England Club, Gauff was unsure whether she would even get into Washington qualifying because she had to use her pre-Wimbledon ranking of 313th to gain entry.
The Washington Open, a combined WTA and ATP event, is one of the key warm-events on the American hard-court swing which culminates in next month’s US Open.
Gauff was the youngest woman to reach the second week of Wimbledon since Jennifer Capriati in 1991.