The highest earning athlete of all time was a Roman charioteer named Gaius Appuleisus Diocles. Adjusted, he earned approximately $15 billion in prize money alone.
Born about 104 A.D., in Lamecum, the capital city of Lusitania, province of Emerita Augusta (modern-day Portugal). Diocles is believed to have started racing at the age of 18 in Ilerda (modern-day Catalonia) and quickly gained a reputation good enough to get himself called up to the ‘big leagues’ in Rome.
The 2nd century star did not make his money through sponsorships or marketing gambits. Instead, the earnings came solely from the prizes he won over a 24-year-long career. Of the 4,257 four-horse races he competed in, Diocles won 1,462 races and was placed in an additional 1,438 races (mostly finishing in second place). That's almost a race competition every other day for 24 years.
At 35,863,120 sesterces in prize money, Diocles's final total far outweighed all others, and has yet to be seriously challenged in the almost 19 centuries since he hung up his reins.
The Romans, as devout followers of sport as any modern society, kept meticulous records with regards to chariot racing; not only the charioteers, their earnings, and their victories, but also stats on the horses as well, which were famed athletes in their own right.
Diocles is among the few to survive this maelstrom of this sport and reach retirement, which he took at age 42. Most did not live past their twenties.