Howarth played some Test cricket with his elder brother Hedley, but most of his 47-Test career did not overlap with Hedley's. He played most of his career as a specialist batsman, captaining the team for 30 of those 47 Tests, and although his batting average of only 32 was not stunning, he did make six Test centuries. Four of those came while he was not captaining the side.
In his debut series as captain, he led the side to their first Test series victory over the West Indies, when New Zealand won the first Test chasing 104 after being 73 for 8, then drew the second Test thanks to 147 from Howarth, and also survived an evenly fought third Test. He also became coach for the New Zealand team in the early 1990s and was coach of the ill-fated tour of South Africa in 1994.
His highest first-class score was 183, for Surrey against Hampshire at The Oval in 1979, "a cultured innings lasting four hours" which helped Surrey to an eight-wicket victory. His best bowling figures were 5 for 32 for Auckland against Central Districts at Auckland in 1973-74.
Howarth played 188 matches for Surrey County Cricket Club in England between 1971 and 1985, and was the first overseas player to captain the club, which he did in 1984–85.