LaJoie began karting at eleven years old. In 1980 he moved into full-bodied racecars and by 1981 had won the sportsman division track championship at the Danbury Fair Racearena in Connecticut. When Danbury closed, he transferred to the Waterford Speedbowl, where he earned Modified Rookie of the Year honors in 1982.
In 1983, LaJoie joined the NASCAR North Tour and immediately claimed the series Rookie of the Year award. He chased the Daytona 500 that year without success, and suffered a frightening crash in the Gatorade Twin 125 qualifying race the following year. Despite the setback, he persevered on the regional circuit and won five races on the NASCAR North Tour in 1985 to claim the series championship — though the title was formally awarded only in 1988 after a lawsuit over a Catamount Speedway disqualification was resolved.
LaJoie made his NASCAR Winston Cup debut at Atlanta Motor Speedway in 1985 and appeared sporadically in both the Cup and Busch Series through the late 1980s and early 1990s. His career trajectory shifted decisively in 1993 when he drove the No. 20 Fina car for Dick Moroso at Talladega in the Busch Series, running with the leaders all afternoon to finish second. The performance earned him additional starts and placed him in contention for a full-time ride.
He drove full-time for Moroso in 1994 in the No. 20 Ford, posting seven top-tens and briefly leading the championship standings before falling to sixteenth at season's end.
The 1996 season proved to be his breakthrough. Hired by BACE Motorsports to drive the No. 74 entry, LaJoie won five races and clinched the Busch Series championship. He repeated the feat in 1997 with another five wins and a second straight title — the only back-to-back Busch Series champion of his era. After slipping to fourth in 1998 and navigating friction with team ownership, he departed BACE at the end of that season.
LaJoie had additional Busch Series success with James Finch's No. 1 operation in 1999, winning the season-opening NAPA Auto Parts 300 at Daytona. He moved to NEMCO Motorsports and then other teams, winning further races through 2002 before his career wound down. In the NASCAR Winston Cup Series, his most sustained run came with Bill Davis Racing in 1995, when he drove thirteen races in the No. 22 MBNA Pontiac before being released. He also ran nine Cup races for Hendrick Motorsports in 1998.
By 2003 his full-time NASCAR career was effectively over, and he shifted toward team ownership, supporting his son Corey's rise through the sport. Corey LaJoie finished second in the 2012 K&N Pro Series East championship to Kyle Larson.
LaJoie worked as a part-time co-host on Sirius Satellite Radio's NASCAR channel and provided television commentary for Busch Series races at the Oregon Raceway Park and Montreal. In June 2010, while working as a spotter on the No. 18 Nationwide Series car, he was suspended indefinitely by NASCAR for violating the substance abuse policy. He was reinstated two months later.
In 2016, LaJoie was inducted into the New England Auto Racers Hall of Fame.
Randy LaJoie's pair of Busch Series championships represents one of the more remarkable late-blooming title runs in NASCAR's developmental series. Winning five races each in 1996 and 1997 with BACE Motorsports, he established himself as a consistent and aggressive competitor on the short ovals that defined that era of the series. His sons Casey and Corey LaJoie have both competed at the Cup Series level, extending the family's presence in American professional motorsport.