Sagadahoc County is part of the Portland–South Portland, ME Metropolitan Statistical Area.

John Smith explored the region in 1614 and reported back to King Charles I, who named the Sagadahoc area "Leethe.

"[5] When the Plymouth Council for New England was dissolved in 1635, 10,000 acres (40 km2) on the east side of the Kennebec River were divided up and granted to private owners.

[5] When King Philip’s War broke out in 1675, the plundering of one house was the only hostile act in Sagadahoc County until August 1676, at which point three settlements were attacked and 53 people taken captive by Native Americans.

The region was almost totally abandoned by settlers, and no permanent settlement was established until 1715, when Arrowsic and Brunswick were founded.

Scotch-Irish Presbyterians began immigrating to the region in increasingly large numbers, though occasional violence persisted until 1759, when the French and Indian Wars ended in Maine.

[5] There were no significant conflicts in Sagadahoc during the American Revolutionary War, despite fear of attack from British cruisers.

Two British armed vessels sailed up the Kennebec River toward Bath, but turned back after being attacked.

During the Civil War the county furnished to the Union forces 2,488 men.

According to the Maine Center for Disease Control and Prevention, Sagadahoc County has the lowest rate of immunization of two-year-olds in the state, at 26%, only a third of the statewide average of 75% and more than 30% lower than the next lowest county in the state.

As of the 2010 United States census, there were 35,293 people, 15,088 households, and 9,869 families living in the county.

[16] In terms of ancestry, 26.9% were English, 16.8% were Irish, 11.8% were German, 8.1% were French Canadian, 6.6% were Italian, 6.5% were Scottish, and 6.4% were American.

[19] Sagadahoc County is a reliable state bellwether, having voted for Maine's statewide winner in every presidential election since 1948.