Dilley was born and raised in Dartford, Kent, England, attended Dartford High School. and played his early cricket for Dartford Cricket Club at Hesketh Park. He trained as a diamond cutter in Hatton Garden before embarking on a cricketing career with Kent County Cricket Club. He played for the Kent Second XI in 1976, aged 17.

He was married and divorced twice. Kent cricketer Graham Johnson was a brother-in-law. He had four children, including Chris Pennell, vice captain of Worcester Warriors rugby team. His youngest son, Jonathan, has also represented Gloucestershire Second XI as a Left Arm Orthodox Spin Bowler.

Early career

Dilley made his first-class debut for Kent at the age of eighteen in 1977, against Cambridge University. He failed to take a wicket, and was not selected again until the following season. He played his second match in June 1978 against the touring Pakistani cricket team, but again failed to take a wicket. It was only in his third first-class match, against Middlesex, that he made his presence felt, taking seven wickets in the match to help his team to a six-wicket win.

Dilley played two more first-class matches that season but took only one wicket. Perhaps more significant was his selection for England Young Cricketers against their West Indian counterparts for two of the three "Tests" and the single one-day game. Real progress in county cricket, however, would have to wait for 1979, when Dilley played 31 senior games for Kent, including a useful effort of 4–41 in the World Cup warm-up match against the New Zealanders. He finished with 49 first-class wickets at an average of 23.48 that season, and already his express pace was attracting attention.

England selection

The England selectors, looking for a young fast bowler for that winter's tour of Australia, took the bold decision of including the 20-year-old Dilley in the squad, and he made his international debut in an ODI against West Indies, played as part of the triangular tournament featuring those two teams and Australia.

Dilley played his first One Day International match on 24 November 1979 against West Indies at Sydney during the 1979/80 Benson & Hedges World Series Cup. The match was won by England by two runs. Dilley was given the new ball and struck in his third over when he claimed the wicket of Desmond Haynes. Rain forced the match to have a revised target and the West Indies needed to score 198 runs in 47 overs to win the match. They fell short and lost the match. Dilley finished with 6–2–21–1 with an economy rate of 3.50 runs per over.

A fortnight later, Dilley appeared in his first Test match, making him the youngest cricketer to play for England in thirty years when he entered the field during the Perth Test at the WACA on 14 December 1979. English captain Mike Brearley showed confidence in Dilley and gave him the new ball at the start of the Australian first innings. Dilley did well during his initial bowling spell, but had to wait until Australia lost their fifth wicket on 127 runs to claim his first Test Wicket. The batsman was Peter Toohey, caught by Derek Underwood for 19. Dilley then claimed his second wicket when Rod Marsh was caught behind by Bob Taylor. Marsh had scored 42 runs and Australia were 219 for 7 at that point. Australia finished 244 all out, and Dilley took figures of 18–1–47–2 with an economy rate of 2.61. However, his debut bowling performance was over-s