Early career Yamaguchi started her professional career in MMA in the lightweight tournament of
Smackgirl-F 2007: The Next Cinderella Tournament 2007 First Stage on , where she defeated Anna Saito by
submission due to a
rear naked choke. Yamguchi won her next fight in Smackgirl, defeating
Saori Ishioka with a
kneebar, forcing Ishioka to tap in the second round in the semi-finals at
Smackgirl-F 2007: The Next Cinderella Tournament 2007 2nd Stage on . On at ''Smackgirl 2007: Queens' Hottest Summer'', Yamaguchi defeated Emi Tomimatsu by
split decision, winning the final of the Next Cinderella Tournament 2007 in the lightweight division. In her last fight with the promotion Smackgirl and her fourth professional match, Yamaguchi was defeated via split decision by
Emi Fujino on at
Smackgirl 7th Anniversary: Starting Over. Her next bout would be at the first event for the promotion Valkyrie,
Valkyrie 01, where she lost by unanimous decision against women's MMA legend
Yuka Tsuji on . It was just the fourth time that Tsuji had been taken to a decision. Yamaguchi rebounded by winning her next two bouts in the inaugural Valkyrie women's featherweight tournament. The first was a unanimous decision win over
Emi Fujino on at
Valkyrie 02 in a rematch of their 2007 bout. The second tournament bout was against Kyoko Takabayashi, whom Yamaguchi was able to defeat with a close split decision at
Cage Force & Valkyrie on , winning the tournament in the process and earning the right to face Yuka Tsuji for Valkyrie's Featherweight championship. At
Valkyrie 04, in her second match against Tsuji, Yamaguchi was once again the
underdog, but she managed to defeat Tsuji with a
rear naked choke in only 76 seconds. This was the first time that Tsuji was defeated by a fellow Japanese fighter and only the second time that she had been defeated in her MMA career. Yamaguchi became Valkyrie's Featherweight Champion in what is considered one of the biggest upsets in the history of
women's mixed martial arts. On , Yamaguchi faced Kyoko Takabayashi in a rematch of the 2009 featherweight tournament final and Yamaguchi's first defense of Valkyrie's featherweight title. Yamaguchi barely retained with a contentious
majority draw in which the dissenting judge scored the fight in favor of Takabayashi at
Valkyrie 08. Yamaguchi faced "Windy" Tomomi Sunaba at
Pancrase Impressive Tour 3 on . She defeated Sunaba by majority decision. Yamaguchi next faced Akiko Naito at
Pancrase Impressive Tour 4 on . She defeated Naito by armbar submission in the first round.
Jewels Yamaguchi faced
Seo Hee Ham in a
Jewels vs. Valkyrie match at
Jewels 15th Ring on . She was defeated by unanimous decision. Yamaguchi faced
Mika Nagano at
Jewels 17th Ring on . She won the fight by split decision. Yamaguchi fought outside Japan for the first time in her MMA career when she faced
Katja Kankaanpää at Botnia Punishment 11 on in Finland. She was defeated by unanimous decision. On , Yamaguchi faced Emi Tomimatsu in a rematch at
Jewels 20th Ring. She defeated Tomimatsu by unanimous decision. Yamaguchi next faced
Megumi Fujii at
Vale Tudo Japan 2012 on . She was defeated by unanimous decision. On , Yamaguchi faced Seo Ye Jung at
Jewels 24th Ring. She defeated Jung by submission due to an armbar in the first round.
PXC In her second fight outside Japan, Yamaguchi faced Patricia Vidonic at Pacific Xtreme Combat 40 on in Guam. She was defeated by split decision. In her sophomore bout in PXC, Yamaguchi faced Gina Iniong at PXC 43 on March 29, 2014. She lost the bout via split decision.
Deep Jewels After Jewels brand was merged into Deep, Yamaguchi faced Yukiko Seki at
Deep Jewels 4 on May 18, 2014. She won the bout via first-round technical submission. In her sophomore bout in the promotion, Yamaguchi faced
Ayaka Hamasaki at Deep - Dream Impact 2014: Omisoka Special on December 31, 2014. She lost the bout via unanimous decision. She was then scheduled to face Ayaka Miura at
Deep Jewels 7 on February 20, 2015. However, Miura withdrew from the bout and was replaced by Miyoko Kusaka. Yamaguchi won the bout via first-minute knockout.
Deep Jewels Featherweight GP and championship In May 2015, Yamaguchi entered into a single-day, four-woman featherweight grand prix which would crown the vacant
Jewels Atomweight champion. She claimed the title by first defeating
Mina Kurobe and then
Satomi Takano.
Return attempt to PXC Yamaguchi was scheduled to face
Tessa Simpson at PXC 52 on March 18, 2016. However, Yamaguchi withdrew from the bout citing an injury and was replaced by her latest opponent
Satomi Takano.
ONE Championship and return to strawweight In late March 2016, it was announced that Yamaguchi had signed with the
ONE Championship, and would debut against
Angela Lee for the inaugural
ONE Women's Atomweight Championship at
ONE Championship: Ascent to Power on May 6, 2016. She lost the bout via unanimous decision.
Post-title shot reign In her sophomore bout in the promotion, Yamaguchi faced Istela Nunes at
ONE Championship: Heroes of the World on August 13, 2016. Yamaguchi lost the bout via split decision.
Return to contention and title rematch Yamaguchi then faced Jenny Huang at
ONE Championship: Light of a Nation on June 30, 2017. She won the bout via second-round submission. She was then scheduled to challenge
Angela Lee in a rematch for the
ONE Women's Atomweight Championship at
ONE Championship: Immortal Pursuit on November 24, 2017. However, Lee was involved in a car accident, the bout was scrapped and Yamaguchi faced Gina Iniong instead. She won the bout via unanimous decision. The title shot against Lee was then rebooked to take place at
ONE Championship: Unstoppable Dreams on May 18, 2018. Despite knocking Lee down multiple times during the bout, Yamaguchi failed to capture the belt in her second attempt, losing via unanimous decision.
Winning streak After the title shot loss, Yamaguchi faced Jomary Torres at
ONE Championship: Destiny of Champions on December 7, 2018. She won the bout via unanimous decision. She was then scheduled to face Bo Meng at
ONE Championship: Call to Greatness on February 22, 2019. However, the bout was scrapped as Meng withdrew from the bout due to an injury. She then faced Kseniya Lachkova at
ONE Championship: A New Era on March 31, 2019. She won the bout via third-round submission. Meng bout was then rebooked to take place at
ONE Championship: Enter the Dragon on May 17, 2019. However Meng bowed out from the bout again due to an injury and was replaced by Laura Balin. Yamaguchi won the bout via first-round submission. She then faced Jenny Huang in a rematch at
ONE Championship: Century on October 13, 2019. She won the bout via unanimous decision.
Losing streak and departure from ONE The bout with Bo Meng was rebooked for the third time to take place at
ONE Championship: King of the Jungle on February 28, 2020. Meng the withdrew from the bout once again and was replaced by Denice Zamboanga. Yamaguchi lost the bout via unanimous decision. Yamaguchi faced Julie Mezabarba as part of an alternate bout for the
Atomweight World Grand-Prix at
ONE Championship: Empower on September 3, 2021. She lost the bout via unanimous decision. Yamaguchi faced Jihin Radzuan at
ONE: Bad Blood on February 11, 2022, as a late notice replacement for Jenelyn Olsim. She lost the bout via unanimous decision.
Post ONE In order to finish her career in her native Japan, she was granted an early release from ONE and subsequently signed with
Rizin Fighting Federation. She dropped to super atomweight division and made her promotional debut against
Kanna Asakura on April 29, 2023, at
Rizin Landmark 5. She lost the bout via unanimous decision. In her next bout, Mei faced Sayako Fujita on December 24, 2023 at Pancrase 340, losing the bout via split decision. == Submission grappling and shoot boxing career ==