Shepherd began racing in 1967 at Hickory Motor Speedway near his home, driving Late Models. After a turbulent start marked by frequent crashes, he won 21 of 29 races in the 1969 season. A difficult decade in the 1970s, complicated by personal troubles, was transformed by a religious experience in February 1975 that Shepherd credited with reviving his focus and ambition. He drove for seventeen different car owners in 1975 alone.
His fortunes changed significantly when he met furniture factory owner Cliff Stewart in 1978. Partnering with Stewart, Shepherd won the 1980 NASCAR Sportsman Series (now the Xfinity Series), taking nine victories and finishing second 21 times. This championship launched him into full-time Winston Cup competition in 1981.
Shepherd made his Winston Cup debut in 1970 at Hickory but did not transition to a full-time schedule until 1981, when he joined Stewart's team driving the No. 5 Pontiac. He won his first Cup race at Martinsville that year, the first Cup win for Pontiac in eighteen years, leading 203 laps. He finished thirteenth in the points standings.
Through the 1980s, Shepherd moved between teams while collecting a respectable record. Driving for Jack Beebe's team in 1986, he won at Atlanta — his second career Cup victory — holding off Dale Earnhardt. At Kenny Bernstein's King Racing in 1987, Shepherd competed in every race, tallying eleven top-tens including seven top-fives.
His most productive period came at Bud Moore Engineering from 1990 to 1991. In 1990, Shepherd led the championship standings after Budweiser 500 and won the season finale at Atlanta for his third career Cup win, finishing fifth in the final points — a career high. He joined Wood Brothers Racing in 1992, finishing second in the Daytona 500 behind Davey Allison.
At the 1993 spring race at Atlanta, racing for Wood Brothers, Shepherd took the lead with twelve laps remaining and won, becoming at age 51 years, four months, and 27 days the second-oldest race winner in Cup Series history, behind only Harry Gant. It was his fourth and final Cup victory. He remained with the Wood Brothers through 1995, producing six top-five finishes in 1994 and a career-best 4,029 points in a single season.
After leaving the Wood Brothers following the 1995 season — when sponsor Citgo sought a younger driver — Shepherd continued competing on a part-time basis through the late 1990s with various teams including Butch Mock Motorsports and Precision Products Racing, with diminishing results.
From 2002 onward, Shepherd fielded his own No. 89 team with minimal sponsorship, frequently starting and parking to conserve resources. Despite the limited budgets, he accumulated hundreds of starts and set multiple longevity records. In 2013, at age 71, he became the oldest driver to start a NASCAR Cup Series race when he ran at New Hampshire Motor Speedway, breaking the previous record set in 1987. He extended the mark again in 2014, racing at age 72.
In the Xfinity Series, Shepherd set similarly remarkable records. He became the oldest driver to lead a Nationwide Series race — at age 70 in 2012 — and repeatedly extended that record through 2017, when he led a lap at Darlington at age 75. He also holds the record for oldest driver to start a race across NASCAR's top three series, having competed at age 77.
In the Xfinity Series he was a prolific winner earlier in his career, taking multiple victories for Ed Whitaker and Lindy White in the 1980s and winning his own team's first Busch Series championship in the lower series. His final Busch win came at Indianapolis Raceway Park in 1988.
Shepherd's identity as a racing Christian ministry figure defined much of his public profile. After his 1975 spiritual awakening — which he described as a transformative moment while returning from Speedweeks after his marriage had collapsed — he integrated his faith into his professional life, serving as a lay minister at tracks and charitable events throughout his career. He has been married to his wife Cindy since 1994 and has six children and ten grandchildren.
In November 2020, Shepherd revealed he had been diagnosed with Parkinson's disease. Despite this, his career stands as one of the most enduring in NASCAR history, defined by resilience, longevity, and an unwavering personal conviction that shaped his approach to both racing and life beyond the track.