Dennis made his debut at age 26 at the 1962 Capital City 300 in the #98 and finished 15th. After three part-time seasons, he attempted a full-time schedule in 1970 and he had five Top 10 finishes for Donlavey Racing. He was given the NASCAR Rookie of the Year award in 1970. He competed full-time for the last time during the following season for Donlavey and had ten Top 10 finishes and a pole position. In 1972 he competed for three teams and earned two Top 5 finishes. He competed in a few races until his last start came in the 1981 World 600.