Mercer's nickname was "Smokin' Joe.” He was apprenticed to trainer Frederick Sneyd, and he won his first British Classic race on Ambiguity in the 1953 Epsom Oaks as an apprentice.
1967 saw two future Classic possibles emerge in Remand and La Mome, but more importantly the Queen (Elizabeth II) first began to send horses to be trained by Dick Hern, thus initiating Mercer's tenure as a Royal Jockey.
Early in May, Remand (owned also by Astor) beat Connaught in the Chester Vase, a recognised Epsom Derby trial, whilst giving the Noel Murless trained colt weight.
He had spent the winter riding in India, could not bring home his earnings due to exchange control issues and decided to buy jewels to export instead.
Everybody who had ridden him had been thrown off – just the once – as if he was saying: ‘I’m the boss.’ The first time I rode him he did the same to me: he just whipped round, dropped his shoulder, dumped me on the ground and stood there looking at me: ‘That’s your place, down there!’”[1]On his return at the end of April, Jimmy Lindley retained the ride on the then stable star Highest Hopes in the One Thousand Guineas.
At the same time news arrived that following the retirement from training of Gordon Richards, his owners Sir Michael Sobell and Lady Beaverbrook would be transferring their horses to Dick Hern.
Jakie Astor negotiated to sell West Ilsley to Sobell and his son-in-law Arnold Weinstock on the understanding that both Hern and Mercer were kept in place as trainer and stable jockey respectively.
Starting the season by winning the Sandleford Priory Stakes at Newbury Racecourse in May and failing to stay the distance in The Oaks trip ran out a good winner of the two furlongs shorter Prix de la Psyche in France.
1975 started slowly for Mercer and at the end of April he had ridden only 5 winners, 3 of them for Peter Cundell plus a victory in the Earl of Sefton Stakes for Herbert Jones on Jimsun.
Mercer did not ride Hern's contender for the Fred Darling Stakes (Garden Party) and instead rode the Queen's filly Joking Apart for Ian Balding into 4th place.
Bill Curling reports in his book 'All the Queen's Horses' that Irish trainer Stuart Murless (a great fan of Mercer's skills) was seeking Joe to ride his filly Nocturnal Spree in the season's first classic the 1000 Guineas.
Soon after he made amends when winning the Duchess of Montrose Handicap at Newmarket, and he rounded off her season when she won the 8 furlong Strensall Stakes over the easier York course.
This promising 2yo followed up with a comfortable victory in Newbury's 7 furlong Donnington Castle Stakes and was expected to complete a hat-trick in the Prix de la Salamandre.
Taking up the running earlier than usual, he entered the Ascot straight 4 lengths clear of that year's Derby winner Grundy, who in a thrilling finish beat Bustino by a half-length.
Willie Carson was retained by Clive Brittain and to him it was a huge shock and his principal owner, Captain Marcos Lemos, said ‘ It was the way the whole wretched business was announced.
On the final day of the Newmarket Craven meeting Mercer rode his first winner for his new stable when Habeebti won a small handicap for owner Charles St George.
Kris then suffered a slight setback and was not seen out again until mid-October when with a lot of stable confidence he won the 6 furlong Rockingham Stakes at York, coming away powerfully from his field to win by an impressive 4 lengths.
Mr Daniel Wildenstein, a rich Paris art dealer, had owned horses for many years and had won the 1976 One Thousand Guineas with Flying Water and the Oaks with Pawneese, both trained in France by Angel Penna.
The jockey's afternoon finished with a last gasp victory on Romara for Harry Wragg beating a hot favourite in Reprocolor (Helena Springfield's filly with Michael Stoute).
Disappointed by this defeat Cecil felt Kris needed more experience and he was given an easy race at Kempton when Mercer rode him to win the Heron Stakes.
Mercer rode seven winners and as well as the aforementioned victories on Le Moss and Kris, his most important success was on Connaught Bridge who won the Nassau Stakes.
The day after The Solent's win Mercer took the Lanson Champagne Stakes on Mr Louis Freedman's Marathon Gold, who at that time appeared the best two-year-old in the yard having won his first race convincingly at Ascot.
Mercer drew a blank on the Friday of Goodwood but finished the meeting with a treble on the aforementioned Connaught Bridge, Suavity (an early 2yo colt who was winning his third race from three starts) and Tahitian King.
Hello Gorgeous was rated only two pounds lower after winning his final race the William Hill Futurity (previously the Timeform or Observer Gold Cup).
A more progressive colt was Light Cavalry, who won a handicap at Newbury mid-May, and then went on to win the King Edward VII Stakes at Royal Ascot.
On the Saturday of the Doncaster meeting Mercer chose to blunt the speed of his main rival Water Mill (Carson/Hern) in the St. Leger Stakes by leading from the start and ultimately winning the final classic by 4 lengths on Jim Joel's Light Cavalry.
As Eddery would ride for the O’Brien stable in the Derby, Mercer was engaged to take over and the partnership smoothly won the Predominate Stakes at Goodwood beating Touching Wood.
By now Mercer was often riding for Charles Nelson and rode a very promising filly called Mahogany to win the Rockfel Stakes, who became strongly fancied for the next year's One Thousand Guineas.
On King George day Mercer did have an interesting winner when Peter Walwyn's, two-year-old debutant, Khozaam won the Granville Stakes in impressive style.
In my carefully considered opinion, it is very hard indeed to believe that a more effective, more stylish or more aesthetically satisfying method exists of persuading a thoroughbred horse to go faster," wrote the journalist John Oaksey[1] Mercer's career total of 2,810 winners was exceeded only by Sir Gordon Richards, Lester Piggott and Doug Smith at the time of his retirement, and subsequently only bettered by Pat Eddery (his niece's ex-husband), Willie Carson and Frankie Dettori.