Tinmouth's 2009 TT debut produced an immediate landmark: she broke the women's lap record set by Maria Costello to become the fastest woman ever to lap the Snaefell Mountain Course, earning a Guinness World Record. She then surpassed her own record in 2010 with an average lap speed of 119.945 mph, claiming a second Guinness World Record. At the 2009 TT she also received the Most Meritorious Performance by a Solo Newcomer Award and the Susan Jenness Trophy for the Most Meritorious Performance by a female competitor — an award she retained in 2010.
Before the TT, Tinmouth had already been accumulating firsts in British domestic competition. She was the first woman to enter and qualify for the British 125GP Championship, the British Supersport Championship, the British Supersport Cup, and the British Superbike Championship. In 2007 she became the first British female to race in World Supersport, and that year she was invited to race for Rizla Suzuki as a teammate to MotoGP rider Cal Crutchlow — the first time a woman had raced a full-blown superbike at that level.
Her 2010 British Supersport campaign delivered another string of records: first woman to lead a British Championship race, first to stand on the podium, and first to win a British Championship race, with a victory in the Supersport Cup at Silverstone. She ended the season with the most points accumulated in the class and, under the rules that year, finished third in the final standings — the highest championship finishing position ever achieved by a female competitor in a British domestic championship at that time.
Tinmouth also won the inaugural UK Electric Bike Racing (TTXgp) Championship in 2010 and finished third in the World Electric Bike Racing Championship the same year, riding an Agni Z2.
From 2011, Tinmouth made the British Superbike Championship her primary focus, entering full-time with the Splitlath Motorsport team. She also established her own BSB outfit, Two Wheel Racing — running under sponsors Hardinge and Sorrymate.com — becoming the first female British Superbike team owner. In 2015, she raced for Honda Racing UK alongside Jason O'Halloran and Dan Linfoot.
Her entry into the BSB field established a Guinness World Record as the first and only woman to compete in the series. The achievement drew international attention and, in 2012, she received the Women's International Film and Television Showcase International Visionary Awards IT Girl award in Los Angeles.
Tinmouth continued to race into the 2020s. For 2020, she announced she would compete in the National Superstock Championship on a Phil Crowe Performance BMW S1000RR, supported by the Ashcourt Group, racing under the number 25. She also competed in Thundersport GB rounds during 2013 alongside her BSB commitments.