McMahons Mill is a boat ramp access point located at Canal Mile 88. 1 on the Big Slackwater section of the Potomac River in Washington County, Maryland, offering a calm, peaceful area ideal for paddling.

The site includes parking and provides access to the scenic Big Slackwater, a lake-like section of the Potomac River created by Dam 4 where canal boats historically transitioned from the C&O Canal to the river.

Note that the area experienced significant flood damage in 2018-2019 and underwent major reconstruction; visitors should check current conditions with the C&O Canal National Historic Park before visiting.