McAllister's Mill was on Rock Creek about a quarter mile north of where it crosses Baltimore Pike. Said to have been built by James Gettys and another man around 1790 on the site of an even earlier mill built by Samuel Gettys in the 1760's, it became the property of James McAllister in 1822. James was a staunch abolitionist, and the mill became an important station on the Underground Railroad. Fugitive slaves would be hidden in the cog pit of the mill, relatively safe from pursuing slave catchers while they rested for the next stage of their journey north.
The McAllister Farm was used as a hospital during and after the battle. A number of soldiers were buried on the grounds, and McAllister put in a claim for $1,200 worth of damages.
Over the years the mill fell into disrepair and eventually collapsed. There are little more than ruins on the site today, which is privately owned.
The millpond dam also deteriorated and the pond that was upstream of the mill is no longer there. This has changed the depth of Rock Creek, which at the time of the battle was probably much more of an obstacle from the Spangler's Spring area south then it is today.