Shepherd began racing Late Models in 1967 at Hickory Motor Speedway near his home. His early career was interrupted by personal difficulties during the 1970s, but a religious conversion in February 1975 reshaped his life and he returned to racing with renewed focus. In 1975 he drove for seventeen different car owners and finished second in the NASCAR Sportsman national championship. He partnered with owner Cliff Stewart in 1978 and won the 1980 NASCAR Sportsman Series title, recording nine wins and 21 second-place finishes that season. The Shepherd-Stewart combination moved to the Winston Cup Series in 1981.
Shepherd had made isolated Winston Cup starts as early as 1970, but his full-time Cup career began in 1981 with Stewart's team. He won his first Cup race at Martinsville that year — Pontiac's first victory in the series in 18 years — dominating the event with 203 laps led. After disagreements ended the Stewart partnership, Shepherd moved to Ron Benfield's team in 1982, recording 13 top-tens and a tenth-place points finish.
He spent much of the early-to-mid 1980s navigating a series of short-term arrangements with different owners, scoring competitive finishes when equipment allowed. His second Cup win came at Atlanta in 1986 while driving for Jack Beebe, holding off Dale Earnhardt in the closing laps after leading 97 laps.
Shepherd's most competitive sustained period came with three consecutive strong operations. Driving the No. 26 Quaker State Buick for Kenny Bernstein's King Racing in 1987, he ran every race for the first time and recorded eleven top-tens with seven top-fives. At Bud Moore Engineering in 1990, he led the Cup points standings after the Budweiser 500 — the only time in his career he held the series lead — and closed the season with his third career win at Atlanta, finishing fifth in the final standings, his career-best.
With the Wood Brothers in 1992, Shepherd finished second in the Daytona 500 behind Davey Allison. In 1993 he became the second-oldest race winner in NASCAR Cup history when he won the spring Atlanta race at 51 years, four months, and 27 days. He equaled his career-best 16 top-tens in 1994 with nine top-fives, finishing sixth in points. After the 1995 season, Shepherd was replaced at the Wood Brothers by Michael Waltrip when Citgo sought a younger driver.
After his Wood Brothers tenure, Shepherd spent the remainder of his Cup career in underfunded equipment, eventually fielding his own No. 89 team. Despite perpetual sponsorship shortages that often forced him to park his car early to conserve tires and fuel, he continued attempting races into his 70s.
In 2013, Shepherd started the Camping World RV Sales 301 at New Hampshire for Brian Keselowski Motorsports at age 71, breaking the previous record for oldest Cup Series starter held by Jim Fitzgerald. He extended that record at a second New Hampshire race in 2014 at age 72. He holds the record for the oldest driver to start a race in any of NASCAR's top three series, set at age 77 in the Xfinity Series.
Shepherd also set and repeatedly extended records in the Xfinity Series (formerly Busch/Nationwide Series) as the oldest driver to lead a lap, first establishing the mark at age 70 in 2012 at Richmond and extending it multiple times through 2017.
In the Busch Series, Shepherd won 14 times between 1982 and 1988, with a best season coming in 1986 when he won four races. His final Busch win came at Indianapolis Raceway Park in 1988.
Shepherd has described his February 1975 religious conversion as the turning point of his life, ending years of personal struggle. He served as a lay minister at NASCAR events throughout his career. He has been married to his wife Cindy since 1994 and has six children and ten grandchildren. In November 2020, he disclosed a diagnosis of Parkinson's disease.
Shepherd's career — spanning more than five decades, four Cup wins, and records for longevity that may stand indefinitely — represents a singular arc of persistence in American stock car racing.