In May 1968, Dancer's Image rallied from last place in a field of 14 to win the Derby by a length and a half. Days later, traces of the drug phenylbutazone were found in the horse's post-race urinalysis, and the colt was disqualified.
Veterinarian Alex Harthill, who had treated many Derby winners, said he had given Dancer's Image a single dose of the medication six days before the race, seemingly enough time for it to clear the horse's bloodstream. The trainer agreed with the approach, and Fuller later said he let the pair handle the horse's well-being.
The ensuing four-year court battle over the disqualification cost Fuller $250,000, he later said. In the end, second-place finisher Forward Pass was awarded the 1968 Derby title.
The son of a former Massachusetts governor, Fuller speculated that racism may have cost him one of his life's great triumphs but told The Times in 1988: "I still don't have a clue to what actually happened."
When Dancer's Image won a pre-Derby race days after the Rev. Martin Luther Jr. was assassinated in Memphis, Fuller sent the winner's share of the purse — more than $60,000 — to the widowed Coretta Scott King.
After the horse was disqualified from the Derby, the media openly speculated that Fuller's support of civil rights had angered wealthy Southern families unaccustomed to a New Englander winning.
"From the outset, it hurt me being the outsider," Fuller said in the Times interview.
He also raised the specter of sabotage by calling Derby security lax. Fuller had described the guard at his horse's barn as "an old fella sitting in a chair and asleep." Race security has since tightened.
Fuller was born March 22, 1923, in Boston, the youngest of four children of Alvan T. Fuller, who was governor in the late 1920s.
The younger Fuller won amateur boxing titles, graduated from Harvard University in 1946 and ran a thriving Boston automobile dealership started by his father.
In the 1950s, Fuller founded Runnymede Farm in North Hampton, N.H., and raised horses that included Mom's Command, which his daughter Abigail rode to New York's Filly Triple Crown in 1985. He sold the farm in 1998.
He is survived by his wife, Joan; seven daughters; one son; 18 grandchildren; and four great-grandchildren.
Fuller eventually sold Dancer's Image, who had a stud career overseas and died in 1992.
On his farm, Fuller kept a sign that read: "Dancer's Image, winner of the 1968 Kentucky Derby."