President Barack Obama’s designation Monday of a new national monument to Harriet Tubman, who escaped slavery on a Dorchester County plantation in 1849, then helped guide scores of other slaves to freedom in the North during the decade before the Civil War, honors a small and unprepossessing African-American woman who played an outsized role in American history.

Mr. Obama’s proclamation sets aside the Harriet Tubman Underground Railroad National Monument near the city of Cambridge on Maryland’s Eastern Shore as a historical preservation site to be administered by the National Park Service. It will be the first such monument ever to commemorate an African-American woman, and the fact that it is being welcomed by state and local lawmakers as well as residents shows how much has changed in the county in recent decades.

Once a place where racial strife was rampant — in 1967, rioting sparked by the city’s refusal to desegregate public accommodations and schools led then-Gov. Spiro T. Agnew to call in the National Guard to restore order — Cambridge elected its first Black mayor in 2008 with biracial support. Since then, it has seen a boom in downtown development and an influx of new residents who have helped the town put its troubled past behind it.