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 (3.5 km) 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 (3.2 km) 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.

Grant County is one of the 36 counties in the U.S. state of Oregon. As of the 2020 census, the population was 7,233, making it Oregon's fourth-least populous county. The county seat is Canyon City. It is named for President Ulysses S. Grant, who served as an army officer in the Oregon Territory, and at the time of the county's creation was a Union general in the American Civil War.