Marion was born in Hampshire. Her father, Ralph, was a farmer and her elder brothers, John and Douglas, were also keen show jumpers. Marion began riding at the age of three, learning on a donkey, but it was in 1960 when her father imported Stroller, an eight-year-old pony, from Ireland. At the age of 16 Marion was coming to the end of her junior career and her father wanted to replace Stroller with a horse, as was customary. Marion was not willing to let him go though, and continued showing on the pony.
This proved to be a shrewd move as her partnership with Stroller proved to be a very successful one. In her second season as a senior Marion won the Queen Elizabeth II Cup—a former international ladies class event—that took place at the Royal International Horse Show. That year she won three Nations Cup events, helping to win the Presidents Cup.
In 1964 Marion won the Hickstead Derby Trial and placed second to Seamus Hayes in the Hickstead Derby itself. The following year Marion rode Stroller to a gold medal in the ladies World Championship at Hickstead.
From there Marion went on to compete at the 1968 Summer Olympics in both the team and the individual jumping events. It was in the individual jumping that she won a silver medal, something that not many had expected her to do. She triumphantly returned from Mexico to win the BBC Sports Personality of the Year and Stroller became somewhat of a celebrity himself.