On his racecourse debut Circus Maximus started the 5/1 favourite in a 23-runner field for a maiden race over seven furlongs at the Curragh on 26 August. Ridden by Ryan Moore he finished fifth, four and three quarter lengths behind the winner Breaking Story, after being hampered in the last quarter mile. On 22 September at Gowran Park the colt was partnered by his trainer's son Donnacha O'Brien in a maiden over one mile on heavy ground and recorded his first victory as he came home two lengths clear of the Ger Lyons trained Army Recruit despite hanging badly to the left in the closing stages. The colt was then sent to England and stepped up in class for the Group 3 Autumn Stakes at Newmarket Racecourse on 13 October and Donnacha O'Brien partnering the stable's best fancied contender Magna Grecia the ride on Circus Maximus went to Seamie Heffernan. After briefly taking the lead a furlong out he was outpaced in the closing stages and finished third behind Persian King and Magna Grecia. Two weeks later Circus Maximus was promoted to Group 1 level when he was one three O'Brien trainees to contest the Vertem Futurity Trophy over one mile at Doncaster Racecourse. Ridden by Wayne Lordan he ran fourth behind Magna Grecia, Phoenix of Spain and Western Australia, beaten a length by the winner after hanging left in the closing stages.
2019: three-year-old season
Circus Maximus began his three-year-old season by starting the 5/4 favourite for the Dee Stakes over ten and a half furlongs at Chester Racecourse on 9 May. Partnered by Moore he took the lead approaching the last quarter mile and despite hanging left yet again he kept on well to win by a length and a quarter from his stablemate Mohawk. Aidan O'Brien commented "Ryan said he was very happy at a mile-and-a-quarter but he wouldn’t mind stepping up if he needed it... this horse could be a Derby possible... He’s lazy, but they're better being a little bit lazy than over-racing." In the 2019 Epsom Derby over one and a half miles on 1 June the colt was partnered by Frankie Dettori and went off the 10/1 sixth choice in a thirteen-runner field. He stumbled at the start but recovered to settle in fourth place but was unable to quicken in the last quarter mile and was eased down by Dettori to come home sixth behind his stablemate Anthony Van Dyck. On 18 June Circus Maximus, equipped with blinkersfor the first time, was dropped back in distance for the Group 1 St James's Palace Stakes over one mile at Royal Ascot when he was ridden by Moore and started at 10/1. Too Darn Hot was made favourite, while the other nine runners included Phoenix of Spain, King of Comedy, Skardu, Shaman and Royal Marine. Racing in heavy rain, Circus Maximus broke quickly and settled in second place behind Fox Champion before taking the lead in the straight. He saw off the challenges of Phoenix of Spain and Too Darn Hot before denying a late charge from King of Comedy on the wide outside to win by a neck. After the race O'Brien explained that the colt had not originally been intended to run in the race and had only been entered when his owners paid a supplementary fee of £45,000 fifteen minutes before the deadline earlier that week. He went on to say "It was a big challenge for the horse. We were worried that the pace was going to be completely different over a mile. We just put the blinkers on him just to sharpen him a little bit. He needed to be focused very quick and he didn't have a lot of time to learn about it". Circus Maximus was matched against older horses in the Sussex Stakes at Goodwood Racecourse on 31 July. After tracking the front-running Phoenix of Spain he took the lead in the straight but was overtaken by Too Darn Hot and beaten half a length into second place. In the International Stakes over ten and a half furlongs at York Racecourse three weeks later, Circus Maximus set the pace before fading in the last quarter mile and coming home seventh behind his stablemate Japan. On 9 September the colt was sent to France and went off at odds of 4.1/1 for the Prix du Moulin over 1600 metres at Longchamp Racecourse. Romanised headed the betting after winning the Prix Jacques le Marois while the other eight runners included Olmedo, Line of Duty (Breeders' Cup Juvenile Turf and Phoenix of Spain. After racing in third place Circus Maximus moved up to dispute the lead 200 metres from the finish and got the better of a closely contested struggle with Romanised to win by a nose. Circus Maximus hung to the left in the closing stages and slightly impeded the runner-up but after an inquiry by the racecourse stewards the result was allowed to stand. Ryan Moore commented "He was the best horse on the day and was always holding off the second. He had a bit more left in the tank. He's obviously well suited to a mile. There were six group 1 winners in the field and you have to be very happy with what he's done." An appeal by Romanised's connections failed to overturn the decision. For his final run of the year Circus Maximus was sent to California to contest the Breeders' Cup Mile at Santa Anita Park on 1 November and started joint-favourite alongside the four-year-old filly Got Stormy. He recovered from a poor start and stayed on well in the straight but never looked likely to win and came home fourth behind Uni, Got Stormy and Without Parole.
2020: four-year-old season
The flat racing season in Britain and Ireland was restructured as a result of the COVID-19 outbreak and Circus Maximus made his first appearance in the Queen Anne Stakes which was run behind closed doors at Royal Ascot on 16 June. With Moore in the saddle he went off the 4/1 favourite in a fifteen-runner field which also included Mustashry, Billesdon Brook, Accidental Agent, Terebellum, Mohaather, Skardu, Plumatic and Duke of Hazzard. After tracking the 40/1 outsider Marie's Diamond, Circus Maximus took the lead approaching the final furlong before being overtaken by the filly Terebellum but rallied to regain the advantage in the final strides and won by a head. Aidan O'Brien, watching the race from his training base in Ireland commented "That's him exactly. He's like a fighter. If he doesn't get his blood up then he doesn't perform. He can be quite lazy so he needs the fast tempo of a mile. He loves getting eyeballed. He's a very good horse. He is very tough, he travels, quickens, and then toughs it out."
