Montgomery made her WTA Tour debut at the 2020 US Open, receiving a wildcard into the women's singles main draw.

[6] In September, she represented the US in the final of the Junior Fed Cup, teaming with Connie Ma to win the doubles match against the Czech Republic and secure victory for the US.

[6] Following the break in the season caused by the COVID-19 pandemic, Montgomery took part in the Western & Southern Open as a wildcard entrant in the qualifying competition, losing in the first round to Sorana Cîrstea.

She and partner Ashlyn Krueger defeated fellow American duo Reese Brantmeier and Elvina Kalieva in three sets.

At the Madrid Open, also as a main draw wildcard, she reached the third round, losing to defending champion Aryna Sabalenka.

[18] At the beginning of the American summer swing, she reached her first WTA 500 quarterfinal at the Washington Open, after receiving a walkover from fourth seed Ons Jabeur.