[4] The county is home to the Patuxent River Naval Air Station and St. Mary's College of Maryland.

Traditionally, St. Mary's County has been known for its unique and historic culture of Chesapeake Bay tidewater farming, fishing, and crabbing communities.

But with the advent of the military bases, growth of an extensive defense contractor presence, and the growth of St. Mary's College of Maryland, as well as increasing numbers of long-distance Washington, D.C. commuters, it has been undergoing a decades-long transformation which has seen the county's population double since 1970.

The settlement of Lord Baltimore's Maryland began with the arrival of passengers from England at St. Clement's Island in the Potomac River in what is now southwestern St. Mary's County on March 25, 1634, and the annual anniversary of this landing is celebrated as Maryland Day.

The passengers arrived in two vessels, the Ark and the Dove that had set sail from the Isle of Wight on November 22, 1633.

This was how a bluff overlooking the nearby St. Mary's River was chosen for numerous reasons, and became the site of the first permanent settlement.

[7] There is a statue in St. Mary's City commemorating this event, along with extensive museums, a reconstructed Colonial town, living history actors, and a replica of the Maryland Dove.

St. Mary's County has some of the oldest extant buildings in English North America.

[8] The United States Colored Troops Memorial Statue is a memorial to the more than 700 African-American soldiers and sailors from St. Mary's County who served among the Union forces during the American Civil War.

The memorial site includes an educational display and special celebrations are held there each year.

In presidential elections, St. Mary's County leans strongly toward the Republican Party.

In 2020, Donald Trump received the lowest percentage for a Republican candidate since Bob Dole in 1996.

Many coastal areas are made up of mixed clay-and-sand cliffs and bluffs, which protect many parts of the county from storm surges, however, there are low-lying coastal areas with coarse sand or gravel beaches or tidal marshlands, as well.

The interior of much of the county is hilly to varying degrees, with forests and agricultural fields.

The county's very extensive waters are mostly brackish, ranging from significant degrees of saltwater in tidal areas that are on or near the Chesapeake Bay, to a greater predominance of freshwater and lower-salt-concentrations in its interior tidal waterways and also further up its bordering rivers.

[14] St. Mary's County waters also are bordered by Virginia's territorial waters in Potomac tributary mouths on the Virginia side,[14] tidal interface zones,[14] and the Chesapeake Bay.

As of the 2010 United States Census, there were 105,151 people, 37,604 households, and 27,084 families residing in the county.

There is also an Old Order Mennonite community in the county that stretches as far south as Loveville.

[30] The county has three naval bases: Numerous state highways serve St. Mary's County, including: Limited local buses are provided through St. Mary's Transit[32] and commuter bus service to Washington, D.C. is provided by MTA.

[33] Southern Maryland Express shuttle services daily schedules to Baltimore (BWI), Dulles (IAD), and Ronald Reagan (DCA).

[citation needed] An expansion pack for the 2008 video game Fallout 3, entitled Point Lookout, is set in a post-apocalyptic version of the titular State Park and its surrounding areas.