Mechanicsville is located in southeastern Hanover County, north of the Chickahominy River. The southern border of the CDP, which follows the Chickahominy, is the Henrico County line. U.S. Route 360 passes through the CDP, leading southwest 6 miles (10 km) to the center of Richmond and northeast 40 miles (64 km) to Tappahannock. Interstate 295, an eastern bypass of Richmond, crosses Mechanicsville from northwest to southeast, with access from exits 41, 38, 37, and 34. U.S. Route 301 forms the northwest border of the CDP, leading south 9 miles (14 km) to the center of Richmond and north 29 miles (47 km) to Bowling Green.
The original rural hamlet of Mechanicsville is in the southwest part of the CDP along US 360, just southwest of I-295 Exit 37. The CDP also includes the neighborhoods or named places of Pearsons Corner, Oak Forest, Rural Point, Borkeys Store, Burnside Farms, Mayfield Farms, Carneals Store, Pole Green, Newman, Dogwood Knoll, Spring Meadows, Brandy Creek Estates, Windy Hill Estates, Ellerson Mill, and Simpkins Corner.
According to the U.S. Census Bureau, the Mechanicsville CDP has a total area of 28.4 square miles (73.6 km2), of which 28.3 square miles (73.3 km2) are land and 0.1 square miles (0.3 km2), or 0.42%, are water. The southern part of the CDP is drained by the Chickahominy River and its tributaries, flowing to the James River, while the northern part is drained by tributaries of the Pamunkey River, part of the York River watershed.