Pompeys Pillar is an unincorporated community in Yellowstone County, Montana, United States and has a postal ZIP code.The town of Pompeys Pillar was founded in 1907 and was named after and situated less than a mile east of Pompeys Pillar National Monument, a 150 ft. tall sedimentary rock formation best known for William Clark's inscription of his name and the date July 25, 1806 on its surface. The site also has significant evidence of human activity spanning an estimated 11,000 years.The town of Pompeys Pillar was first planned out as a railroad station within the Huntley Project, an irrigation project managed by the United States Bureau of Reclamation.A Catholic church and a Union Congregational church once existed in Pompeys Pillar. The Northern Pacific Railroad connected the town to Billings, about 29 miles southwest. A post office currently operates in the community.