Fort Mahan, situated on an isolated hill belonging to Dr. William Manning, was built to guard the approaches to Benning's Bridge which crosses the Anacostia River.
Fort Mahan was irregularly shaped with nine sides and a perimeter of 354 yards. This fort was located less than mile east of Benning's Bridge and commanded the approaches to the bridge from Bladensburg via the Eastern Branch Road, and from Upper Marlboro by way of Benning Road.
Although the fort was completed and ready for assignment of a garrison in December 1861, the engineers were far from satisfied with it. Over the next four years it underwent extensive repair and modification. The major problem with the fort was that it sat atop a rounded hill and approaches to it were not visible from the parapet. To fix this problem 400 yards of rifle pits were constructed to cover its approaches. Structures were built that would provide a full range on which to attack oncoming troops (bastionets). These structures were projected from three angles of the exterior slope of the ditch (counterscarp) to provide reverse and covering fire for the exterior slopes. After Early's raid in 1864, the engineers laid abatis and constructed rifle trenches toward Anacostia River in order to prevent a flanking movement from that side.
Fort Mahan was one of the later forts to be abandoned. At the end of the Civil War, Fort Mahan was named among the 11 works that occupied commanding positions and should be retained for a skeleton defense of the city. However, it was abandoned and reverted back to the Manning family. Mary Manning claimed damages in 1864, and in 1865 bought the abatis around the fort for $31 at public auction.
Structures that were built at Fort Mahan included a guard house, at least two barracks, officers' quarters, a mess house, stable, and some sheds.