Mount Vernon is a city in Grant County, Oregon, United States. Its post office was established in 1877 and named after a black stallion that belonged to settler David W. Jenkins. The stallion's stable, a small stone building, is still standing in a field on the north side of U.S. Highway 26 about 2.2 miles east of the main intersection in town. In 1915 the ranching community had a population of 60 and a hotel at a hot springs resort approximately 2 miles north. The population had grown to 451 by the 1950 census. There are a few motels and restaurants in Mount Vernon. The population was 527 at the 2010 census.