Tennell began skating when she was two years old, even though she learned to walk late and had to wear orthotics in her shoes to correct a pronation problem in her feet.

[2][3] Tennell skated most of her life at a rink in Buffalo Grove, Illinois, where she also gave lessons to young skaters before beginning her own training, even after competing nationally.

[26] Competing at the 2018 Olympics was a goal, although Tennell told reporters throughout the season that she preferred to focus on one competition at a time because thinking about making the U.S. team could be overwhelming for her.

[17] Tennell's first event of the season was the 2017 Philadelphia Summer International in early August 2017; she came in first place with a successful triple Lutz-triple toe in both of her programs, which earned her an invitation to Skate America.

She competed against three-time U.S. national champion Ashley Wagner and former World Championship medalists Gabrielle Daleman of Canada, Satoko Miyahara of Japan, and Alena Leonova of Russia.

[30] Tennell was pleased with her performances at Skate America but went home with the goal of working on fine-tuning both her programs for Nationals, including earning Level 4 scores on both step sequences.

[24][31] Longman, who reported that Tennell had successfully completed every triple jump she had attempted for three competitions, also said that she handled the pressure of possibly qualifying for the Olympics with no outward sign of nervousness.

[17] Reporter Christine Brennan called choosing Tennell instead of Wagner, a more well-known, established, and experienced skater despite her fourth-place finish at Nationals, "a gamble"[32] for U.S.

[13][33] United Airlines paid for her mother and two brothers to travel to PyeongChang to watch her compete, after the company discovered that the family had set up a GoFundMe page to raise money for the trip.

[33] She received lower scores for her choreography, skating skills, and transitions compared to more experienced skaters like Kaetlyn Osmond from Canada and Carolina Kostner from Italy.

[47] In her short program, Tennell chose music from the 2014 film Lucy, a song called "Rebirth" performed by Hi-Finnesse and Egyptian-British singer Natacha Atlas.

[48] For her free skate, which Richaud also choreographed, she used music from three versions of Romeo and Juliet, in order to make the program modern and to emphasize Tennell's strength, speed, and emotion.

NBC Sports reported that she "scored a big upset"[45] over two-time world champion and Olympic silver medalist Evgenia Medvedeva of Russia; it was Tennell's first senior international title.

[45] Reporter Karen Price said that she "put on a display of power, grace and her trademark clean routine"[46] that showcased her technical prowess and tenacity.

[49] Figure skating analyst Tara Lipinski stated that although Tennell was not yet at the same level as Medvedeva or Zagitova, her performance at the Autumn Classic demonstrated a strong start to the season, as well as a dramatic improvement in her music choices, choreography, and intention behind each movement.

[73][74] Tennell's short program music included "a fast-paced medley of [Russian musician] Kirill Richter's staccato piano compositions",[74] which demonstrated a lighter, more "fun-loving" side of her personality.

Despite not being able to bend her arm the previous morning, Tennell came in first place in the short program over defending champion Alysa Liu, and Mariah Bell.

For Tennell's triple Axel, Zakrjsek worked with her on a different entry pattern and increasing the time she was in the air after her takeoff in order to help her completely rotate the jump consistently.

She told Philip Hersh from NBC Sports that she was frustrated about not reaching her goals or progressing fast enough, as well as by her failure to add the triple Axel.

[103] Her placement, along with Chen's fourth, secured two berths for the United States at the 2022 Winter Olympics, with the possibility of a third, and earned three places at the following year's World Championships.

[109] In December 2021, Tennell announced on social media that she was withdrawing from the 2022 U.S. Championships, due to the same foot injury that had plagued her all season, making her ineligible to compete in the Olympics unless she filed a petition for a place on the U.S team.

[110] Tennell called it "the hardest decision of my life", wished her fellow skaters good luck at Nationals and in Beijing, and vowed that she would return to competition.

[110] After waiting several months to allow her foot to heal, Tennell traveled to France in July to work with her longtime choreographer, Benoit Richaud, in La Garde.

[111] She and Richaud created a short program that Tennell thought showed a different side to her skating and hoped would "bring more awareness to a social issue that I feel is very important in today's society".

[111] She was able to participate in the 2022 MK John Wilson Trophy in Sheffield, a replacement event for the Cup of China, which was cancelled due to travel and quarantine restrictions caused by COVID and would provide her with a bye into the 2023 U.S. Championships.

She came in first place in the short program, with 68.84 points, successfully executing her opening triple Lutz-triple toe combination jump and a double Axel; she also earned level 4s on all of her spins.

[117] Tennell explained that her final spin errors occurred "because I got a little excited", adding that "we're going to take that in stride and move forward for the long program".

[123][124] She said on social media that "I had a little mishap while warming up my step sequence during training" and "caught an edge on a bracket and managed to break my ankle in a funky fall".

[125] Going into the 2024-2025 season, after five months off the ice, Tennell reported that she felt "100% healthy" and continued full-time training with Benoit Richaud via FaceTime, phone calls, and "strategically planned visits" while living on her own in West Orange, New Jersey.

She rarely fell in competition, which helped her earn high technical element scores; she credited it to the technique her first coach, Denise Myers, taught her.