[3] His new owner was considered an eccentric character[4] who gave most of her horses names consisting of one word with seven letters (for example Bustino, Terimon, Niniski, Mystiko, Petoski), as this was the most common form for Derby winners.
Despite his uncooperative attitude, he was moved up in class for the Group One Middle Park Stakes at Newmarket Racecourse and, ridden by stable work-rider Brian Proctor, finished fourth behind Tudenham.
Ridden by Joe Mercer he started a 16/1[8] outsider but won by length from the undefeated Dewhurst Stakes winner Lunchtime, who had been the ante-post favourite for the Guineas.
Later that month Boldboy was moved up in distance for the Blue Riband Trial Stakes over eight and a half furlongs at Epsom and finished second to Gospill Hill when carrying seven pounds more than the winner.
Boldboy returned to France for the Group One Prix Jacques Le Marois at Deauville Racecourse in August and finished fourth behind Kalamoun, Rose Laurel and Sparkler, three and a half lengths behind the winner.
[7][8] Boldboy began his third season in the six furlong Abernant Stakes at Newmarket in April and led from the start and won by four lengths from the three-year-old Cawdor, to whom he was conceding twenty-four pounds.
Starting at odds of 15/8,[8] the gelding was restrained by Mercer in the early stages, before overtaking Owen Dudley two furlongs out and holding off the challenge of the French-trained four-year-old El Rastro to win by a length (surviving a lengthy Stewards Enquiry).
[10] On his debut as a six-year-old, Boldboy recorded his first success in over a year when he won the Abernant Stakes for a second time, beating Honeyblest by a head after leading from the start.
He produced one of his best performances in defeat in the Waterford Crystal Mile at Goodwood in August, when he was beaten a short head by Free State, carrying nineteen pound more than the runner-up.
[11] During his 1976 campaign, Boldboy broke the record for earnings by a British-trained gelding, surpassing the mark set by the stayer Petty Officer who had been retired a year earlier.
He began with a third win in the Abernant Stakes and then finished second by a head when attempting to concede nine pounds to the four-year-old colt Duke Ellington in the Victoria Cup at Kempton.
He ended his season by recording his most important win to date with a "most impressive and decisive" two and a half length victory over Hillandale when even money favourite[8] the Vernons Sprint Cup.
In the Kiveton Park Steel Stakes at Doncaster he was beaten three quarters of a length by the filly Green Girl, to whom he was conceding twenty-three pounds.
[6] There was no International Classification of European three-year-olds in 1973: the official handicappers of Britain, Ireland and France published their own ratings of horses which had raced in those countries.
[12] The independent Timeform organisation rated him on 126, ten pounds behind their top-rated three-year-old Thatch but ahead of Mon Fils, Morston and Peleid, the colts who won the three races of the Triple Crown.
[15] In their book, A Century of Champions, based on the Timeform rating system, John Randall and Tony Morris described Boldboy as "one of the best– and best-loved– geldings ever to race on the Flat".
[16] Dick Hern said that Boldboy was "a wonderfully versatile horse, top class over six furlongs and a mile, a very good doer, and one of the soundest who ever came my way.