Maktoum bin Rashid Al Maktoum
He subsequently worked as stable jockey for Jack Colling, Dick Hern, Henry Cecil and Peter Walwyn. During his spell at Cecil's yard he won his only British flat racing Champion Jockey's title in 1979. The most successful horse Mercer rode during his career was Brigadier Gerard, winner of 17 of his 18 races between 1970 and 1972. He retired as a jockey in November 1985. He then worked initially as a jockey's agent before accepting a job as racing manager for Maktoum bin Rashid Al Maktoum in 1987. He retired from that in January 2006.