In August 2019, Montgomery played in the Girls' Singles at the US Open, where she reached the third round, losing to fellow-American Katrina Scott.[4] In September, she represented the US in the final of the Junior Fed Cup, teaming up with to win the doubles match against the Czech Republic and secure victory for the US.[5] In December, she won the "18 and under" title in the 2019 Orange Bowl.[6]
Montgomery reached the quarterfinals of the 2020 Australian Open Girls' Singles tournament in January, and in March she won her first ITF tournament, a $25,000 event in Las Vegas.[1] As of August 2020 she was at No. 5 in the junior world rankings.[4]
Following the break in the season caused by the COVID-19 pandemic, Montgomery took part in the Western & Southern Open as a wildcard entrant, losing in the first round to tenth seed Sorana Cirstea.[7] The following week, she received a wildcard into the 2020 US Open—her first senior Grand Slam appearance.[4] She lost in the first round to Yulia Putintseva.[8]