- Dillon is a city located in Beaverhead County, Montana, United States. It is the county seat and the largest city in the county. As of the 2010 census, the population of Dillon was 4,134. The city is named after Sidney Dillon, a president of the Union Pacific Railroad. Dillon is known for its scenic beauty and outdoor recreational opportunities, including fishing, hunting, hiking, and skiing. The University of Montana Western is also located in Dillon, making it a center for higher education in the region.