Dennis made his debut at the age of 26 at the 1962Capital City 300 in the No. 98 and finished fifteenth. After three part-time seasons, he attempted a full-time schedule in 1970 and he had five top-ten 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-ten finishes and a pole position. In 1972, he competed for three teams and earned two top-five finishes. He competed in a few races until his last start came in the 1981 World 600.