Rainey began his career racing in the A.M.A. Grand National Championship, a series that encompassed four distinct dirt track disciplines plus road races.[3] In 1981, he finished the Grand National season as the 15th ranked dirt track racer in the country.[5] Following his success in the Novice 250cc roadrace class, Kawasaki hired him to compete in the 1982 AMA Superbike Championship as a teammate to the then defending National Champion Eddie Lawson.[3] The following year, Lawson moved to the Grand Prix circuit and Rainey took over the role of leading rider, earning the 1983 National Championship for Kawasaki.[3]
In 1984, he accepted an offer to ride for the newly formed Kenny RobertsYamaha racing team in the 250cc class of the Grand Prix World Championship.[3] A less than successful season (1 podium and difficulty push-starting the bike) saw him returning home in 1985 to join the Maclean Racing team in U.S. 250 and Formula 1 classes, and then on to the American Honda team from 1986 to 1987 where he raced Superbike and F1.[3] It was during the 1987 Superbike National Championship that his intense rivalry began with Kevin Schwantz as the two battled it out for the title.[3] Rainey won the Championship, but the fierce rivalry between the two competitors was just beginning. So intense was their rivalry that they continued their battle during the 1987 Transatlantic Trophy match races in which they were supposedly teammates competing against a team of British riders.[3]
In 1988 Rainey returned to Europe, again joining Team Roberts Yamaha, this time in the premier 500cc division riding the YZR500.[3] His arch-rival Schwantz followed him to Europe, signing to race the 500cc class for Team Suzuki. The two would continue their rivalry on race tracks all across Europe, driving each other to higher levels of competitiveness. In 1988, Rainey won his first world championship race at the British Grand Prix.[2] Also in 1988, he and his Team Roberts Yamaha teammate Kevin Magee won the prestigious Suzuka 8 Hours endurance race in Japan.[6] In the 1989 campaign, Rainey finished second overall behind Eddie Lawson, winning three races and placing on the podium in every race that he finished.[2]
From 1990 to 1992, Rainey hit his stride earning three consecutive 500cc crowns for Yamaha.[2] Rainey was involved in a hard-fought campaign with Kevin Schwantz while seeking his fourth-consecutive title in 1993. He was leading the championship by a margin of 11 points over Schwantz, and leading the GP when he suffered his career-ending crash at the Italian Grand Prix in Misano.[3] He slid into the gravel trap at high speed, breaking his spine against the raked surface designed as a safety feature for car racing. The injury handed the title to his great rival Kevin Schwantz.[2] Rainey's injuries rendered him permanently paralyzed from the chest down.
After turning to Williams team owner and quadriplegicFrank Williams for advice, Rainey later became the team manager for MarlboroYamaha for a few years.[3] After the 1995 season, Schwantz retired from the Grand Prix circuit, partly due to nagging injuries and partly because losing the one great rival that had fired his competitive intensity made him view his own mortality much more clearly.[7]
Rainey refused to give up racing despite his disability and raced a hand-controlled Superkart in the World SuperKart series based in Northern California.
After several seasons of poor decisions by Daytona Motorsports Group, the organization that operated AMA-sanctioned road racing beginning in 2009, a dispute with Dorna/Infront in 2013 regarding AMA Superbike coverage on shared FIM weekends led to a lack of media coverage for that year's Superbike round at WeatherTech Raceway Laguna Seca, as well as the Harley-Davidson XR1200 round at Indianapolis Motor Speedway in both 2013 and 2014, the AMA transferred the commercial rights to AMA-sanctioned road racing to Rainey and his business partners at the end of 2014. The MotoAmerica era of AMA-sanctioned road racing began in 2015.[11]
Rainey rode his 1992 YZR500 at the 2022 Goodwood Festival of Speed. Yamaha specially modified the bike by moving the rear brake and gear shift controls to the handlebars.[12][13]