[3] Branstine was born in Irvine, California, to an American father, Bruce, and a Canadian mother, Carol Freeman, from Toronto.

[8] In March 2015, she played her first professional tournament, losing to Karolína Stuchlá in the first round of the $10k in Gainesville, Florida.

[11] In March, Branstine captured her second junior singles title with a victory over Ann Li at the G4 in Newport Beach, California.

[16] In December, Branstine made it to the semifinals in singles and to the quarterfinals in doubles at the Eddie Herr ITF G1 in Bradenton, Florida.

[18] In January at the Australian Open, Branstine reached the third round in girls' singles and captured the doubles title with Bianca Andreescu.

[1] She started representing Canada officially in March and played her first tournament as a Canadian at the G1 in Carson, California at the end of the month, where she went on to win both the singles and doubles titles.

They upset Kristina Mladenovic and Anastasia Pavlyuchenkova in the first round, before falling to the top seeds, Ekaterina Makarova and Elena Vesnina.

[23] The following week at the Tournoi de Québec, she advanced with Andreescu to her first WTA Tour doubles final in which they were defeated by the top-seeded Tímea Babos and Andrea Hlaváčková.

Branstine returned to the Texas A&M Aggies for the 2024 postseason, contributing to the team winning their first NCAA Championship.