The Mariners have been a member of the Cape Cod Baseball League since 1930, when the team first began play in Harwich. In the league's Modern Era, the team has qualified for the postseason on 22 different occasions. Its first appearance came in 1968, when Harwich dropped the championship series, 3-1, to the Falmouth Commodores in what would become the first of Falmouth's four consecutive titles between 1968 and 1971.
Harwich made the postseason three times in the 1970s, reaching the championship series in 1978 and 1979, losing both times to the Hyannis Mets in four games.
Finally, in 1983, Harwich broke through, winning its first Cape Cod Baseball League championship in the Modern Era. The Mariners finished the regular season with only the third-best record in the league, but eliminated the Hyannis Mets, 2-1, to earn a berth in the championship series. Harwich prevailed over the top-seeded Cotuit Kettleers in five games, taking the series to win the crown.
After finishing the season with the best record in the league in 1987, Harwich defeated the Yarmouth-Dennis Red Sox to win its second league title in five years.
The Mariners saw little success after that, qualifying for the postseason only once, in 1997, over the next several seasons when they were swept by the Wareham Gatemen in the championship series.
After an 11-year postseason drought, the Mariners stormed the Cape League in 2008, and sweeping the Cotuit Kettleers in dramatic fashion to claim the Cape Cod Baseball League championship in 2008. The Mariners swept the Falmouth Commodores in 2011 to win their second championship in four years.
Since 2008, the Mariners have since reached the playoffs in all but three subsequent seasons. However, they had reached the championship round only once prior to 2024, losing to Cotuit in 2019. They won the East Division three times in the regular season since 2008, with titles in 2012, 2014 and 2016.
In 2024, Harwich earned the No. 3 seed in the East playoffs and defeated No. 2 seed Chatham in a ninth-inning comeback to reach the East Championship Series. Once there, the Mariners swept No. 1 seeded Yarmouth-Dennis in two games and proceeded to win the championship in three games over Bourne.
