On her racecourse debut Hydrangea ran unplaced in a maiden race over six furlongs at the Curragh on 17 July and then finished second in a similar event at Galway Races nine days later. In a minor event over seven furlongs at the Curragh she started the 4/9 favourite she recorded her first success as she led from the start and won "comfortably" by two and a quarter lengths from Madame Cherie. Hydrangea was then stepped up in class and finished second in her next three races. She was beaten a head by Rhododendron in the Group 2Debutante Stakes, a short head by Intricately in the Group 1 Moyglare Stud Stakes and two and a quarter lengths by Rhododendron in the Fillies' Mile. On her last run of the year she was sent to California to contest the Breeders' Cup Juvenile Fillies Turf at Santa Anita Park but came home last of the fourteen runners behind New Money Honey.
2017: three-year-old season
On 8 April at Leopardstown Racecourse Hydrangea started at odds of 5/1 for the 1,000 Guineas Trial Stakes in which her opponents included Promise To Be True, Winter and Intricately. Ridden by Padraig Beggy she disputed the lead from the start and stayed on well in the final furlong to win by a head from Winter. Hydrangea was beaten by Winter in her next four races, finishing tenth in the 1000 Guineas, third in the Irish 1000 Guineas, third in the Coronation Stakes and fourth in the Nassau Stakes. On 9 September Hydrangea, ridden by Wayne Lordan, started a 20/1 outsider for the Matron Stakes over one mile at Leopardstown. The field was a very strong one: Winter started favourite while the other contenders included Qemah, Persuasive, Roly Poly, Wuheida, Rhododendron and Intricately. After racing in second place she looked beaten a furlong out but rallied "gamely" in the closing stages to win by a head from Winter. After the race Aidan O'Brien said "I'm delighted for the filly and I'm delighted for Wayne. She is Winter's work partner all year, and maybe she turned around and said Hydrangea, you can have this one today!." At Chantilly Racecourse on 1 October Hydrangea was beaten ahead by Rhododendron in the Prix de l'Opera over 2000 metres. Ryan Moore took the ride Ascot Racecourse 20 days later when the filly was stepped up in distance for the British Champions Fillies & Mares Stakes over one and a half miles on soft ground. The Prix Vermeille winner Bateel started favourite ahead of Journey with Hydrangea the 4/1 third choice in the betting. The other seven runners included Coronet, The Juliet Rose, Left Hand, Horseplay and Alyssa. Hydrangea tracked the leaders, took the lead in the straight and fought off a challenge from Bateel before drawing away to win by two lengths. O'Brien commented "She’s by Galileo and they will not stop, their will to win is incredible. She pulled out more and it was Ryan’s idea to run her as he thought there was a chance she’d get the trip. We weren’t sure, but she did". In the 2017 World's Best Racehorse Rankings Hydrangea was given a rating of 119, making her the 59th best racehorse in the world and the third-best three-year-old filly, level with Winter and behind Enable and Lady Aurelia.
2018: four-year-old season
On her four-year-old debut, Hydrangea started favourite for the Lanwades Stud Stakes at the Curragh in May but was beaten into second place by the Mick Channon-trained Opal Tiara. In her next two races she was unplaced in the Duke of Cambridge Stakes and then finished last of seven in the King George VI and Queen Elizabeth Stakes. After a two month break the filly returned to run for a second time in the British Champions Fillies & Mares Stakes on 20 October and came home fifth of the eleven runners behind Magical.