Undefeated at atomweight, Lee successfully defended the belt five times before vacating the title when she retired from the sport in September 2023.

[16] Following the win, Lee dropped out of college in Hawaii (she was studying business) and moved to Singapore to join Evolve MMA.

[17][18] In her next fight, she used a rear naked choke hold to beat Mona Samir at ONE Championship: Odyssey of Champions on September 27, 2015, in the first round.

Lee's third professional fight was at ONE Championship 33: Pride of Lions on November 13, 2015,[19] and she submitted Natalie Gonzales Hills with a twister in the opening round in what commentators described as a potential submission of the year contender.

[23] Next, she faced Lena Tkhorevska at ONE Championship 36: Spirit of Champions on December 11, 2015,[24] and won with a rear naked choke in the second round.

Lee's first fight of 2016 was against Rebecca Heintzman at ONE Championship 39: Tribe of Warriors on February 20, 2016, and she won with a second round neck crank.

[29][30] Lee won the fight by unanimous decision to become the inaugural ONE Women's Atomweight Champion, and received a $68,200 SGD bonus for her performance.

Commentators speculated that her new contract made Lee the highest paid teenager in the history of MMA, male or female.

[35][36] On March 11, 2017, Angela Lee defeated Jenny Huang of Taiwan and retained the Atomweight World Championship title.

[46] At ONE Championship 96: Masters of Destiny, Angela Lee suffered another defeat at the hands of Michelle Nicolini, via unanimous decision.

[54] At ONE Fight Night 14 on September 30, 2023, Lee announced her retirement from mixed martial arts and vacated the title during the event.

[62][63] The Lees shut down their family-owned MMA training facility in Waipahu, Hawaii, and said it would remain permanently closed.

[64] On September 19, 2023, Lee wrote in an article on the Players' Tribune that her 2017 car crash in Hawaii was a suicide attempt, and also revealed that her sister Victoria, who died in 2022 at the age of 18, had taken her own life.