Tosaki started his career as a jockey for the
Ohi Racecourse, part of the
National Association of Racing, in 1998. He rode and won his first race on April 12 of that year, when he rode Miyasan Yashima to victory. Tosaki rode his first
JRA race on June 26, 2005, when he rode Spring Lagos at a maiden race in
Fukushima Racecourse, who finished at 10th place. He won his first JRA race on July 8, 2007, which also happened to be on his 27th birthday, at
Hanshin Racecourse with Yamakatsu Tigger. Tosaki rode his first JRA graded race with at the 2007
March Stakes, where he finished 14th. He later went on to win the
Tokyo Derby with Dream Sky for the second year in a row after he won the 2007 race with Ampersand. He also won his first JpnI race with Furioso on that year's
Teio Sho with Furioso. These wins contributed to him being named the Best Jockey of the Year in the 2008 NAR Grand Prix. On March 25, 2009, Tosaki won his 1,000th NAR victory after winning the Keihin Hai with . On June 30, Tosaki won the Teio Sho with Furioso for the second time, beating JRA horses such as
Kane Hekili, , , and . Tosaki also won his first JRA graded race with when they won the
Musashino Stakes. Tosaki won his first JRA Grade I race when he won the
Yasuda Kinen with in 2011. Tosaki won his 2,000th NAR victory with Greco, when they won a race at
Funabashi Racecourse on March 14, 2012. Tosaki transferred from Ohi to JRA from March 1, 2013 after earning a JRA jockey license earlier that year. Tosaki had earned his JRA license on his third attempt. Following this, he has Tosaki won his first post-transfer JRA Grade Race with . Tosaki won the held at
Sonoda Racecourse on December 12. In 2014, Tosaki became the JRA leading jockey, with his major win including
Gentildonna at the
Arima Kinen. Tosaki won his first race abroad while participating in the Longines International Jockey Championship held at
Happy Valley Racecourse in Hong Kong on December 9, 2015, where he rode True Comment and finished in a
dead heat. He ultimately finished the championship at third place after earning a total of 13 points. In 2016, Tosaki became the JRA leading jockey for the third year, and for the first time in JRA history without earning a single penalty point. On June 29, 2019, Tosaki won an allowance race with Ecris l'Histoire, which became his 1,000th JRA victory. On November 4, 2019, Tosaki suffered a fall during the
JBC Ladies' Classic held at Urawa Racecourse, resulting in him an open fracture of his right elbow, taking him out of the race for six months. He returned to racing in late May of the following year. Tosaki won his first win after his fall on August 16, 2020, at the
Sekiya Kinen held at
Niigata Racecourse with . Later that year, on December 6, he won his first dirt Grade I race with at the
Champions Cup. On February 20, 2021, Tosaki won the
Saudi Derby with substituting for
Joel Rosario, who was originally supposed to ride the horse but was barred from entering Saudi Arabia. He followed this victory up by finishing second in the
Riyadh Dirt Sprint riding behind another Japanese-trained horse, . However, he and Chuwa Wizard finished 9th in the main race of that day, the
Saudi Cup. One month later, Tosaki participated in that year's
Dubai World Cup Night with Matera Sky and Chuwa Wizard. He rode Matera Sky in the
Dubai Golden Shaheen, who finished 12th, but finished second at the
Dubai World Cup, which was the best performance for a Japanese-trained horse in that race since 2011. On January 20, 2024, Tosaki achieved the 52th jockey ever to have had 10,000th starts in a JRA race when he raced a maiden race at Nakayama Racecourse with Lien du Miracle, and later won his 1,500th JRA victory after winning the Sagamiko Tokubetsu held at
Tokyo Racecourse with Omega Wing on June 16. That same year, he won the
Satsuki Shō with
Justin Milano. Following the victory, he commented in a post-race interview while in tears, referring to the horse's former training partner who had died in a racing accident a week prior, that "Fujioka told me a lot about the horse's conditions. I think Kota gave us the last push and is very happy about it. I just wanted to say 'thank you Kota. You did well'". On December 22, Tosaki rode
Regaleira on the
Arima Kinen as her main jockey,
Christophe Lemaire, had opted to ride
Urban Chic for that race. After going neck and neck against
Shahryar, Regaleira beat the horse by a nose, becoming the first three-year-old filly to win the race since did in 1960, and Tosaki himself won his second Arima since Gentildonna in 2014. On January 26, 2025, Tosaki rode
Danon Decile for the
American Jockey Club Cup as the horse's main jockey,
Norihiro Yokoyama, was to ride for that race. In that race, Matenro Leo and contested the lead in the final stretch, before Danon Decile passed both horses from the outside, winning the race. This was the first time since
Special Week in 1999 where the Derby winner from the previous year won the race. On March 22, Tosaki became the 17th jockey ever to win 1,600 JRA races following him winning the
Flower Cup with . On April 5, Tosaki won his first Grade I race abroad with Danon Decile at the Dubai Sheema Classic. == Major wins ==