The pair beat Nicole Melichar-Martinez and Ellen Perez 6-4 6-7 (5-7) 10-5 in Sunday’s final, which was decided by a tiebreak.
At 18 years and 154 days, Gauff is the second youngest player in WTA history to reach the top of the doubles rankings after Switzerland’s Martina Hingis, aged 17 years and 251 days.
“Being No. 1 is pretty cool,” Gauff told reporters after winning her second doubles title with Pegula and fifth overall.
“I have no words. I didn’t really know it was going to happen this week and what I had to do, but (Pegula) told me yesterday. It didn’t make me more nervous though. I think if it was singles, I would have been more nervous.”
Gauff and Pegula, who are also America’s top singles players, were down 3-0 in the first set of Sunday’s final in Toronto but won six of the next seven matches to take an early lead.
They had six championship points in the second set, but Melichar-Martinez and Perez held on to force a decisive tiebreak. The match looked balanced at 6-5, before Gauff and Pegula retired to clinch the title.
While most players tend to focus on singles or doubles throughout their career, Gauff has competed in both disciplines and reached the singles and doubles finals at Roland Garros this year.
“As a kid, I’ll be honest with you, I didn’t even know there were separate rankings for singles and doubles,” she said. “But when I went on tour and realized I could do doubles well…I wanted to. I mean, who wouldn’t want to be No. 1 in anything?”