County Maps US

St. Mary's County, Maryland

A county in Maryland, ranked 12 of 24 by population.

CountyFIPS 24037

Within Maryland
St. Mary's County, highlighted, with the areas around it. Boundaries generalised from the 2025 Census cartographic boundary file; neighbour fills carry no data. The dot marks Leonardtown, the county seat, located via the 2025 Census Gazetteer.

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Population
116,692
+2,700 (+2.4%) since 2020
Land area
358.6
square miles. A further 53.1% of the total area is water
Population density
325
people per square mile, computed rather than published
County seat
Leonardtown
Seat of county government.
Established
1637
388 years ago
Largest city
Leonardtown town
Population 5,033.

Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025; Wikidata (CC0), Wikidata. Retrieved 30 August 2026.

Satellite view

Aerial imagery of St. Mary's County with its boundary drawn on top. A map shows you the shape; this shows you what is inside it, and which side of the line a place falls on.

Aerial view of St. Mary's County, Maryland from the USGS National Map, with its boundary outlined
Imagery: USGS National Map, USGS Imagery Only, public domain, requested from USGS by your browser when you reach this point on the page, at 739 by 800 pixels. The boundary is the 2025 Census cartographic boundary file, reprojected to Web Mercator to match the imagery. You can also open the USGS National Map viewer.

Where St. Mary's County ranks

Position among the 24 counties of Maryland, and among all 3,144 US counties and county-equivalents. Computed from the figures on this page.

MeasureValueIn MarylandNationally
Population116,69212th of 24552nd of 3,144
Land area358.6 sq mi16th of 242,660th of 3,144
Population density325 per sq mi13th of 24408th of 3,144
Median household income$119,4467th of 2456th of 3,143

Full data for St. Mary's County

Every figure below is attributed to the Census Bureau product it came from, with the vintage of that product.
MeasureValueSource and vintage
Population116,692Vintage 2025 Population Estimates
Population in 2020113,992Vintage 2025 Population Estimates
2020 estimates base?113,772Vintage 2025 Population Estimates
Land area358.6 sq mi2025 Gazetteer Files
Water area405.7 sq mi2025 Gazetteer Files
Total area764.3 sq mi2025 Gazetteer Files
Population density?325 per sq miComputed: Vintage 2025 estimates divided by 2025 Gazetteer land area
Median household income$119,446American Community Survey 2020-2024 5-Year Estimates
Median home value$407,600American Community Survey 2020-2024 5-Year Estimates
Median age37.2American Community Survey 2020-2024 5-Year Estimates
Households42,436American Community Survey 2020-2024 5-Year Estimates
Housing units46,385American Community Survey 2020-2024 5-Year Estimates

Sources: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026; US Census Bureau, Gazetteer Files, 2025 Gazetteer Files, released 10 September 2025; US Census Bureau, American Community Survey, American Community Survey 2020-2024 5-Year Estimates, released 29 January 2026. Population density is computed here, not published: it divides the Vintage 2025 population estimate by the 2025 Gazetteer land area, which are two different Census products. Retrieved 30 August 2026.

How the population changed

The Census Bureau publishes the components behind an estimate, not just the total. For the year to July 2025:

Births minus deaths
+368
1,314 births, 946 deaths
Net migration
+97
More people moved in than moved out.

Source: US Census Bureau, Population Estimates Program, Vintage 2025 Population Estimates, released 26 March 2026. Retrieved 30 August 2026.

The 6 areas bordering St. Mary's County

Ordered by the length of the border actually shared, which the Census Bureau publishes in its county adjacency file and which no other county reference presents.

Bordering areaStateShared borderPopulation
Charles CountyMaryland34 mi176,593
Calvert CountyMaryland28 mi94,484
Westmoreland CountyVirginia25 mi19,715
Dorchester CountyMaryland20 mi33,628
Northumberland CountyVirginia19 mi12,557
Somerset CountyMaryland12 mi24,973

Source: US Census Bureau, County Adjacency File, 2025 County Adjacency File, released 15 September 2025. Retrieved 30 August 2026.

All 24 counties of Maryland

Open the interactive Maryland county map, or jump straight to any of them.

Data on this page last rebuilt 30 August 2026. How these figures are produced.