The pair defeated Nicole Melichar-Martinez and Ellen Perez 6-4, 6-7 (5-7), 10-5 in Sunday’s final, decided by match 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, who was 17 years and 251 days old.
“Being No. 1 is pretty cool,” Gauff told reporters after winning her second doubles title and fifth overall title with Pegula.
“I have no words. I didn’t really know it was coming this week and what I had to do but (Pegula) told me yesterday. However, it didn’t make me more nervous. I think if it was single I would have been more nervous.”
Gauff and Pegula, who are also America’s top singles players, fell 3-0 in the first set of Sunday’s final in Toronto but won six of their next seven games to take an early lead.
They had six championship points in the second set, but Melichar-Martinez and Perez held on to force a crucial tiebreak. The match appeared to be even at 6-5 before Gauff and Pegula pulled away to take the title.
While most players focus on either singles or doubles during their careers, Gauff has competed in both disciplines, reaching the singles and doubles finals at this year’s French Open.
“As a kid, to be honest, I didn’t even know there were separate rankings for singles and doubles,” she said. “But once I got on tour and realized I could do well in doubles… I wanted to. I mean, who wouldn’t want to be number 1 in anything?”