Early career In early 2021, Sundell was ranked the number one female featherweight (57.153 kg) Muay Thai practitioner in the world by the
WBC Muaythai at 16 years of age. She has notable victories over decorated champion
Sawsing Sor Sopit, Ponpan Por.Muengpetch, and male fighter Bongsak Sitpholek.
ONE Championship In her ONE Championship debut at
ONE: Full Circle on 25 February 2022, Sundell faced Australia's Diandra Martin. She won the fight by TKO in round 3. This was her first fight in four-ounce
MMA gloves, which are smaller than the usual gloves used in Muay Thai. Sundell made her kickboxing debut against Milana Bjelogrlić at
ONE Friday Fights 18 on 26 May 2023. She won the fight by unanimous decision. Sundell made her first Women's Muay Thai Strawweight World title defense against
ONE Women's Atomweight Muay Thai World Champion Allycia Rodrigues at
ONE Fight Night 14 on 29 September 2023. She retained the championship by a third-round technical knockout. She also earned her second $50,000 Performance of the Night bonus. In the second title defense, Sundell faced
Natalia Diachkova on 4 May 2024 at
ONE Fight Night 22. At the weigh-ins, Sundell came in at 126.5 lb (after coming in at 127.7 lb in for an unofficial check two and a half hours into the weigh-ins), 1.5 pounds over the strawweight limit for a title fight. As a result, Sundell was stripped of the title and only Diachkova was eligible to win it. She won the fight via technical knockout in the second round.{{cite web |last=Joseph |first=Mike |date=2024-05-04 |title=ONE Fight Night 22 Results: Smilla Sundell Folds Natalia Diachkova, Abdullaev Shocks Amir On 19 August 2025, it was reported that Sundell was removed from the ONE roster, her weight having exceeded its heaviest women's weight class.{{Cite web |date=2025-08-19 |title=ONE Championship releases six fighters from its roster == Mixed martial arts career ==