The 80-year-old, whose name has not been released, was flown to Ruby Memorial Hospital with head injuries.
West Virginia State Police were called to Fields Creek Road at 9:30 a.m. where they found a car had struck the mower.
Trooper Tony Taylor said his investigation determined the woman was crossing Gladesville Road from Fields Creek Road to mow property across the county route.
The accident was near the Preston and Monongalia County line.
There were no other injuries reported.
Taylor said no charges will be filed.