Denness became a tall, stylish right-handed batsman. He received his county cap in 1965 and Kent won the County Championship in 1970 for the first time since 1913. He succeeded Colin Cowdrey as Kent captain at the beginning of the 1972 cricket season, having often substituted for Cowdrey when he was on Test duty. Under his captaincy, the club won the John Player League three times (1972, 1973, 1976), the Benson & Hedges Cup twice (1973, 1976), and the Gillette Cup once (in 1974; also his benefit season). Denness was a Wisden Cricketer of the Year in 1975. After 5 years as captain, the club voted to replace him as captain by Asif Iqbal at the end of the 1976 season.