St. Mary's City is easiest by car. From Washington, D.C. or the Capital Beltway, follow routes south through southern Maryland toward Leonardtown and St. Mary's County. The drive usually takes around two hours, longer with weekend traffic.
Public transport is limited, so do not plan on arriving casually by train. Once there, the historic area is walkable, and a bicycle is useful if you want to explore the quiet roads nearby.