Mainardi began gymnastics when she was four years old because her older sister also did gymnastics.[2]
Mainardi made her international debut at the 2021 Junior Pan American Championships and won a bronze medal with the Argentine team behind the United States and Brazil.[3] Then at the 2021 South American Junior Championships, she won the silver medal in the all-around behind teammate Isabella Ajalla and a gold medal in the team event.[4]
Mainardi competed at the 2022 South American Youth Games, winning gold in the all-around, vault, and floor exercise. She also won silver with the Argentine team.[5] At the 2022 Junior Pan American Championships, she won a bronze medal with the Argentine team.[6] Individually, she placed fifth in the all-around, eighth on the uneven bars, and seventh on the balance beam. In the floor exercise final, she won the silver medal behind American Tiana Sumanasekera.[7] Then at the Argentine Championships, she won a bronze medal in the all-around.[8] She finished her 2022 season at the Top Gym Tournament in Charleroi, Belgium, winning bronze medals on the vault and floor exercise and placing seventh all-around.[9]
Mainardi was selected to compete at the 2023 Junior World Championships in Antalya, Turkey alongside Emilia Acosta and Isabella Ajalla, and the team finished in fifth place.[10] Mainardi qualified for the all-around final and finished twelfth.[11] In the vault final, Mainardi won the gold medal with an average score of 13.383.[12][13] She also competed in the uneven bars final where she finished sixth.[14]