The island was named after Major John Mason who was granted the island in recognition for his military services in the 1637 Pequot War in nearby Mystic.

The island is approximately 600 acres (240 ha) in area overall, being 1.2 miles (1.9 km) in length north-south and 0.8 miles (1.3 km) in width east-west and is formed by solid granite bedrock.

[2] There is a 47-acre (19 ha) nature preserve located in about the middle of the island.

This nature preserve features a large salt marsh with surrounding woods that can be accessed by multiple trails.

In the southern end of the island there is a large fresh water pond that was formerly used for harvesting ice.