he made his debut for northumberland in the 1963 minor counties championship against durham. thewlis played minor counties cricket for nothumberland from 1963 to 1981, which included 152 minor counties championship appearances. he scored 2,225 runs in total, taking 100 catches as well. he made his list a debut against lincolnshire in the 1971 gillette cup. in this match, he was dismissed for 7 runs by sonny ramadhin. he made a further list a appearance against somerset in the 1977 gillette cup. in this match, he dismissed for 19 runs by ian botham. he also captained northumberland on a number of occasions.