Since Kilby Mill Falls is on Persimmon Creek; and not on a creek named Kilby, I reasoned there must have been a gristmill at this location operated by a family named Kilby. A quick google search turned up an application form for the National Register of Historic Places
The James Henry and Rachel Kilby House is located just south of the trailhead on Tumbling Waters Lane. The document verifies that the Kilby family operated a gristmill (on Persimmon Creek). Although mill stones and timbers existed as late as 1961, no evidence of the mill now remains.