With her late husband Fred, Rimell saddled the winners of a record-equalling four Aintree Grand Nationals - ESB, Nicolaus Silver, Gay Trip and Rag Trade.
She took on the training licence after being widowed and won the 1983 Champion Hurdle at Cheltenham with Gaye Brief.
"She had a great life and passed away in peace," said grandson Mark Rimell.
"She was a great woman and left her mark in a great sport," he told the Press Association.
Mercy Rimell also won the Stayers' Hurdle with Gaye Chance in 1984 and the 1987 Arkle Trophy with Gala's Image.
She took over the licence from former champion jockey and trainer Fred in 1981, and retired from training in 1989.
However, she remained in the sport as an owner and breeder.
BBC racing correspondent Cornelius Lysaght
Mercy and Fred Rimell were among the most formidable training combinations jump-racing has known.
This was very much a team effort, and a hugely successful one. From their base outside Worcester, names such as ESB, Rag Trade, Woodland Venture, Royal Frolic and the great champion hurdler Comedy Of Errors won a string of major prizes.
After retirement, Mercy became a racehorse owner, when in her late 80s, famously pleading for a low weight for her Grand National hope Simon because, as she put it: "I'm old, and won't be around to see him for much longer." The horse ran in 2007 and 2008.
Mercy Rimell, the first woman to train a Champion Hurdle winner, has died aged 98.