Costello has achieved a career high singles ranking of world number 187, on October 10, 2022, and a career-high doubles ranking of world number 218 on August 1, 2022.
Costello reached the junior US Open and singles and doubles semifinals for the two events, making her the first tennis player to reach all four junior semifinal events at a major.
Costello has made eight finals on the ITF Circuit including one title.
Costello plays for the Yale Bulldogs women's tennis team.
Costello was born in Manhattan Beach, California, and began playing tennis at age three. She was inspired by her grandmother, who was a former tennis player. Costello's father, Sean, is a lawyer and her mother, Cindy, is a homemaker.
Costello has three older siblings, Sean Jr., Ryan, and Casey. Sean Jr. and Ryan played college tennis at the University of California, Berkeley and the University of Southern California, respectively.
Costello had a successful junior career, reaching a career-high ITF junior ranking of world number 1 on 24 January 2019. She won the 2017 Orange Bowl and the 2019 Easter Bowl.
Costello was also the runner-up at the 2019 Australian Open and French Open. She reached the semifinals of the 2018 US Open and 2019 US Open.
Costello represented the United States in the Junior Fed Cup in 2018 and 2019. She helped lead the United States to the title in both years.
Costello played college tennis at the University of California, Berkeley. In her freshman year, she was named the Pac-12 Freshman of the Year.
Costello was ranked #1 singles player in college tennis after going 12–1 in singles this year. She finished the season with 16–7 overall singles record and 13–6 doubles record.
Costello turned professional in 2022 after graduating from college. She has since won one W25 tournament and reached the semifinals of two WTA 125 tournaments.
In the 2022 NCAA Division I Women's Tennis Championships Singles draw, Costello was the top overall seed and won the title by defeating No. 2 seed Peyton Stearns of Texas.
Costello made her WTA Tour main draw singles debut at the 2022 San Jose Classic, where she lost in the first round to fourth seed Madison Keys.
| W | F | SF | QF | #R | RR | Q# | P# | DNQ | A | Z# | PO | G | S | B | NMS | NTI | P | NH |
To avoid confusion and double counting, these charts are updated at the conclusion of a tournament or when the player's participation has ended.
|Tournament |2023 |
|---|---|
|Australian Open |Q1 |
|Tournament |2023 |
|---|---|
|Australian Open |1R |