2012–2020
Sullivan joined Capital Area Runners after graduating in 2012. She debuted in the marathon at Grandma's Marathon in 2015, finishing 9th in 2:35:37 to qualify for the 2016 U.S. Olympic Trials Marathon, where she placed 20th in 2:41:18.[10]
In 2017 she sustained a hamstring injury in a car accident, which sidelined her for an extended period. She returned to the marathon at the Richmond Marathon in 2019, where she ran 2:42:35 — at that point the slowest marathon of her career — but the result nonetheless met the qualifying standard for the 2020 Olympic Trials.[11] At the Trials in Atlanta, she placed 145th in 2:45:57.[12]
2020–present
Following the 2020 Olympic Trials, Sullivan began training under Andrew Gerard, Director of Track & Field and Cross Country at George Mason University.[13] Later that year she competed at The Marathon Project in Arizona, finishing 20th in 2:33:27.[14]
In 2021 she placed 15th at the 2021 Boston Marathon in 2:33:22, finishing as the fourth American woman.[15]
In 2022 she won the Cherry Blossom Ten Mile Run (52:32), becoming the first local runner to win the women’s title since 1983,[16][17] placed third at Grandma’s Marathon (2:26:56), and finished sixth at the 2022 Chicago Marathon (2:25:14), her first top-10 at a World Marathon Major.
At the January 2023 USA Cross Country Championships, Sullivan finished eighth in the senior women’s 10 km, which placed her as an alternate for the World Cross Country Championships in Bathurst, Australia. [18]
In April 2023 she placed 10th and was top American at the 2023 London Marathon in 2:24:27.[2] Shortly afterward, she announced her first professional contract with Brooks.[19]
At the 2023 World Athletics Championships in Budapest, she finished 58th after competing with a fractured kneecap that worsened during the race.[20] Owing to ongoing recovery from that injury—and a COVID-19 infection the week of the race—she withdrew from the 2024 U.S. Olympic Trials Marathon in Orlando.[21]
In October 2024, Sullivan ran a lifetime best of 2:21:56 at the 2024 Chicago Marathon, finishing seventh overall and first American.
In 2025, she was again the top American at the 2025 London Marathon (10th, 2:29:30). In September, she placed fourth at the 2025 World Athletics Championships marathon in Tokyo (2:28:17); this performance marked the highest finish by an American woman in the event since Amy Cragg won bronze in 2017.[22] She concluded the year at the New York City Marathon in November, finishing 11th in 2:28:34.[23] At the conclusion of the 2025 season, she was ranked No. 2 in the U.S. in the marathon by Track & Field News.[24]
Sullivan opened her 2026 season at the Houston Half Marathon on January 11, running a personal best of 1:08:44 to finish 12th.[2]
