Deegan began riding dirt bikes at the age of seven and received her first Trophy Kart as an eighth-birthday gift, launching an off-road career that spanned more than a decade. She won in her very first race in the SXS Stadium Series Trophy Kart class in 2009, and by 2013 had become the first female driver to claim a Lucas Oil Off Road Racing Series championship when she won the Junior 2 Karts class. She added the Modified Kart Regional Championship in 2015 and the national title the following year, also taking the 2016 Lucas Oil Off Road Driver of the Year award. She is one of four women in LOORRS history to win a class championship.
Deegan began her transition to asphalt in 2016 with legends cars for Rev Racing and was immediately enrolled in NASCAR's Drive for Diversity program. She received the NASCAR Diversity Young Racer award in 2017 and was named to the NASCAR Next class in both 2017 and 2018, serving as the youngest and only female member each time. Toyota welcomed her into its driver development program.
Her NASCAR debut came in 2018 with Bill McAnally Racing in the K&N Pro Series West. At the Dirt Track at Las Vegas Motor Speedway, she became the first woman in NASCAR history to win a pole at the K&N level and then converted that speed into her first career win on September 29 at Meridian Speedway in Idaho, passing teammate Cole Rouse on the final lap. It was only the second win by a woman in any NASCAR touring series race, following Shawna Robinson's victory in the now-defunct NASCAR Dash Series. Deegan closed 2018 as the West Series Rookie of the Year.
She returned to K&N competition in 2019 and added two more series wins, including a second consecutive last-lap pass victory at the Las Vegas Dirt Track to open the season, and a third win at Colorado National Speedway. She also made her ARCA Menards Series debut with Venturini Motorsports that year.
Deegan switched from Toyota's development program to Ford Performance at the end of 2019 and signed with DGR-Crosley — renamed David Gilliland Racing in January 2021 — for a full-time Truck Series schedule beginning in 2021. She drove the No. 1 Ford, becoming the first woman to score a Truck Series top-ten finish outside a superspeedway when she finished seventh at World Wide Technology Raceway at Gateway in August 2021. She was voted the 2021 Truck Series Most Popular Driver.
Her Truck Series debut had actually come in October 2020 at Kansas, where she finished 16th — the best result in series history for a woman in a debut race. Also in 2020, she led a career-best 16 laps at Talladega before finishing 24th in a full ARCA Menards Series season with DGR-Crosley that yielded the series Rookie of the Year award and a third-place championship finish.
In 2022, Deegan made her NASCAR Xfinity Series debut for SS-Green Light Racing at Las Vegas, finishing thirteenth on the lead lap — the best result in series history for a woman making her Xfinity debut. She continued making occasional Xfinity starts in subsequent seasons and signed a multi-year deal with AM Racing for 2024 full-time Xfinity competition, though the pairing struggled. The team posted only four finishes of twentieth or better through seventeen races and the two parties parted ways mid-season, with Joey Logano filling her seat for the Chicago street race.
In October 2024, Deegan announced she would compete full-time in the Indy NXT Series for HMD Motorsports in 2025, driving the No. 38 entry. She finished fourteenth in her debut at St. Petersburg and ended the season fourteenth in the championship with a best finish of eleventh at Laguna Seca. She returned to stock car racing in 2026, signing a full-time deal with Bill McAnally Racing for the ARCA Menards Series West along with part-time Truck Series appearances with McAnally-Hilgemann Racing.
Deegan developed a reputation for aggressive on-track behavior rooted in her off-road background. All three of her K&N Pro Series West wins involved contact with another car, and the third saw her spin out her own teammate for the position. Series veterans noted her combative approach, though Deegan embraced the aggressive identity and attributed it to her family's roots in motocross culture.
Deegan grew up in Temecula, California and attended Rancho Christian High School. She currently splits time between California and Mooresville, North Carolina. Her father Brian Deegan is a co-founder of Metal Mulisha and a multiple X Games gold medalist. Her younger brother Haiden Deegan is a professional motocross racer and the 2023 AMA SMX 250 champion. Deegan announced her engagement to dirt track racer and former NASCAR driver Chase Cabre in October 2023.