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.

[5] 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.

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.

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 blinkers for 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.

In the International Stakes over 10+1⁄2 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.

Romanised headed the betting after winning the Prix Jacques Le Marois while the other eight runners included Olmedo (Poule d'Essai des Poulains), Line of Duty (Breeders' Cup Juvenile Turf) and Phoenix of Spain.

[13] 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.

The flat racing season in Britain and Ireland was restructured as a result of the COVID-19 pandemic and Circus Maximus made his first appearance in the Queen Anne Stakes which was run behind closed doors at Royal Ascot on 16 June.

"[15] On 29 July Circus Maximus made his second attempt to win the Sussex Stakes at Goodwood and led from the start and maintained his advantage into the straight but was overtaken by Mohaather in the final strides and beaten three quarters of a length into second place.

[16] He reappeared for the Prix Jacques le Marois over 1600 metres on heavy ground at Deauville Racecourse on 16 August and came home third behind the three-year-olds Palace Pier and Alpine Star.

In the Queen Elizabeth II Stakes at Ascot in October he never looked likely to win and came home tenth of the fourteen runners behind The Revenant after fading badly in the closing stages.

He raced in mid-division before finishing strongly, but after briefly struggling to obtain a clear run in the straight he failed by a neck to overhaul his less fancied stablemate Order of Australia.

[19] Five days after his run at Keeneland, it was announced that Circus Maximus would be retired from racing at the end of the year and would stand as a breeding stallion at the Coolmore Stud at an initial fee of £20,000.