Lebanon County is a county in the Commonwealth of Pennsylvania, United States. As of the 2020 census, the population was 143,257. Its county seat is the city of Lebanon. It lies 72 miles northwest of Philadelphia, which is the nearest major city. The county is part of the South Central Pennsylvania region of the state. The county was formed from portions of Dauphin and Lancaster counties in 1813, with minor boundary revisions in 1814 and 1821.