Cohasset was first seen by Europeans in 1614, when Captain John Smith explored the coast of New England. The area was first settled in 1670 and became a town separate from Hingham in 1770. Previously, what is today the town of Cohasset was known as Hingham's Second Parish. The town's name came from the Algonquian word "Conahasset", meaning "long rocky place". Much of the land was originally granted to the "Conahasset Partners".

At a special town meeting of January 1670, the shares in the new town were apportioned and divided among the new proprietors, many of whom were large Hingham landowners. The largest number of shares—35—went to Hingham Town Clerk Daniel Cushing, with the second largest (25) to Reverend Peter Hobart, Hingham's minister. Others receiving large grants were: Capt. Joshua Hobart, Peter Hobart's brother (18 shares); Lieut. John Smith (15 shares); Ensign John Thaxter (16½ shares); and deacon John Leavitt (with 14½ shares). The layout of the town was distinctive. Many lots were laid out in long narrow strips, facilitating more lots having road frontage, and avoiding back lots.

Cohasset was originally part of Suffolk County, and when the southern part of the county was set off as Norfolk County in 1793, it included the towns of Cohasset, Hingham and Hull. In 1803 Hull and Hingham opted out of Norfolk County and became part of Plymouth County, leaving Cohasset as an exclave of Norfolk County.

According to the United States Census Bureau, the town has a total area of 31.5 square miles (82 km²), of which, 9.9 square miles (26 km²) of it is land and 21.6 square miles (56 km²) of it (68.56%) is water. It is bordered on the west by Hingham, on the northwest by Hull, on the north and northeast by Massachusetts Bay and on the east and south by Scituate. Cohasset is approximately twelve miles east of Braintree and twenty miles southeast of Boston.

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