Lamar is a city in Johnson County, Arkansas, United States. The population was 1,605 at the 2010 census, up from 1,415 at the 2000 …Lamar is a city in Johnson County, Arkansas, United States. The population was 1,605 at the 2010 census, up from 1,415 at the 2000 census. Lamar is located in southeastern Johnson County in the valley of Cabin Creek. U.S. Route 64 is Lamar's Main Street and leads northwest 5 miles to Clarksville, the county seat, and south 4 miles to Knoxville. Interstate 40 crosses US 64 2 miles south of Lamar at Exit 64 and leads west 64 miles to Fort Smith and southeast 19 miles to Russellville. Little Rock is 95 miles southeast of Lamar via I-40.