2013: two-year-old season

Mikki Isle began his racing career by finishing second to Atom in a race for newcomers over 1600 metres at Hanshin Racecourse on 7 September. On his next appearance the colt won a maiden race at Kyoto Racecourse, completing the 1600 metre course in a record time of 1:32.3. He ended the year at Nakayama Racecourse on 14 December by winning the Hiiragi Sho over the same distance.^[5]

2014: three-year-old season

Suguru Hamanaka, who rode Mikki Isle in most of his races

Mikki Isle began his second campaign in the Grade 3 Nikkan Sports Sho Shinzan Kinen at Kyoto on 12 January in which he started the 0.6/1 favourite and won by half a length from Win Full Bloom.^[6] On 1 March the colt started odds-on favourite for the Grade 3 Arlington Cup at Hanshin and recorded his fourth consecutive victory as he came home three and a half lengths clear of Tagano Grandpa.^[7]

On 11 May the colt was moved up to Grade 1 class and started the 0.9/1 favourite for the NHK Mile Cup at Tokyo Racecourse with his seventeen opponents included Atom, Rosa Gigantea (Spring Stakes) and Shonan Archive (Grade 2 New Zealand Trophy). He took the lead soon after the start, repelled the sustained challenge of Horai Akiko and held off a wave of late challengers in the closing stages to win by a neck from Tagano Burg.^[8] After the race Suguru Hamanaka said "being sent off as heavy favorite, I'm relieved with the outcome. Unusually, he was tense in the gate and didn't break as well, but once he spurted for the lead he was faster than the others. I knew the others were closing in fast, and prayed that he would hold on to his lead. This horse has incredible speed and character. I hope he will go on to enjoy a fantastic career".^[9] On 8 June, over the same course and distance, Mikki Isle was matched against older horses in the Yasuda Kinen. Racing on soft ground he led for most of the way but tired badly in the straight and came home sixteenth of the eighteen runners behind Just A Way, beaten twelve lengths by the winner.

After a lengthy summer break Mikki Isle returned for the Grade 2 Swan Stakes over 1400 metres at Kyoto in which he again faced older rivals. Starting the 1.5/1 favourite he won by half a length and a nose from the five-year-olds Sunrise Major and Fiero.^[10] Three weeks later the colt started favourite for the Mile Championship at the same track but after taking the lead in the straight he faded rapidly and was eased down by Hamanaka and finished unplaced behind Danon Shark. The colt was the beaten favourite again on his final start of the year, coming home seventh to Real Impact in the Grade 2 Hanshin Cup on 27 December.

In the JRA Awards for 2014, Mikki Isle placed fourth behind Snow Dragon in the poll for Best Sprinter or Miler receiving 12 of the 285 votes.^[11]

2015: four-year-old season

On his 2015 debut Mikki Isle was beaten a nose by the six-year-old Daiwa Maggiore, to whom he was conceding five pounds in weight, in the Grade 3 Hankyu Hai over 1400 metres on heavy ground at Hanshin on 1 March. At the end of the month the colt was dropped back in trip for the Grade 1 Takamatsunomiya Kinen over 1200 metres at Chukyo Racecourse in which he produced a strong late run down the centre of the track but was beaten half a length and a nose behind the Hong Kong challenger Aerovelocity and Hakusan Moon.^[12] In his second attempt to win the Yasuda Kinen on 7 June Mikki Isle came home fifteenth of the seventeen runners behind Maurice.

Mikki Isle returned in autumn for the Sprinters Stakes over 1200 metres at Nakayama on 4 October. After tracking the leaders he went to the front 100 metres from the line but was overwhelmed by fast finishers in the final strides and was beaten into fourth place behind Straight Girl, Sakura Gospel and Ukiyono Kaze. For his final run of the year the colt was sent to Sha Tin Racecourse for the Hong Kong Sprint in which he finished seventh to the locally trained Peniaphobia after fading in the straight.^[13]

2016: five-year-old season

Mikki Isle's trainer Hidetaka Otonashi

Kohei Matsuyama took over the ride from Hamanaka for Mikki Isle's first three races of 2016. The horse made his seasonal debut in the Hankyu Hai on 28 February and recorded his first victory for fifteen months as he won by three quarters of a length from the gelding Omega Vendetta.^[14] Mikki Isle's second bid for the Takamatsunomiya Kinen saw him start second choice in the betting behind Big Arthur on 27 March. After racing just behind the leaders he took the lead in the straight but was overtaken by Big Arthur in the last 100 metres and finished second by three quarter of a lengths.

After a break of more than six months. Mikki Isle returned to action for a second tilt at the Sprinters Stakes on 2 October. He took the lead soon after the start, maintained his advantage into the straight and looked likely to win before being caught on the line and beaten a head by Red Falx. Hamanaka was reunited with Mikki Isle for the Mile Championship at Kyoto on 20 November. The horse was made the 4.9/1 third choice in the betting behind Satono Aladdin (Swan Stakes) and Isla Bonita (Satsuki Sho) in an eighteen-runner field which also included Neorealism, Snow Dragon, Danon Shark, Young Man Power (Fuji Stakes), Fiero and Sunrise Major. Mikki Isle led from the start, fought off a sustained challenge from Neorealism and prevailed by a head from Isla Bonita despite hanging left in the closing stages. The racecourse stewards held an inquiry into the interference cause by the winner's failure to keep to a straight course but left the result unaltered.^[15] Hamanaka reported "I apologize for interfering with other horses in the stretch. Mikki Isle ran really well and I was able to race him in the front as planned. The horse was in good form and performed his best in the race".^[16]

On his final racecourse appearance, Mikki Isle started favourite for the Hanshin Cup on 24 December but came home sixth of the fifteen runners behind the three-year-old Shuji.

In the JRA Awards for 2014, Mikki Isle was named Best Sprinter or Miler receiving 169 of the 291 votes to take the award ahead of Maurice (52 votes), Big Arthur (29) and Red Falx (23).^[17]

In the 2016 World's Best Racehorse Rankings Mikki Isle was given a rating of 118, making him the 74th best racehorse in the world.^[18]