Pimmit Run forms south of Leesburg Pike (State Route 7) in Pimmit Hills. From Pimmit Hills, Pimmit Run flows northeast through Pimmit Run Stream Valley Park and then under the Dulles Toll Road into Devon Park and Chesterbrook Gardens neighborhoods. Pimmit Run continues on its northeastern course meandering through Pimmit Run Stream Valley Park. At Westmont, the stream is joined by Little Pimmit Run before paralleling George Washington Parkway and flowing southeast into Arlington County. The stream parallels Chain Bridge Road as it bisects the Arlington Bluff and empties into the Potomac just downstream of the bridge.
The stream was named for John Pimmit who in 1675 was an overseer for William Fitzhugh(1651-1701). John was naturalized a citizen in 1679, and died by drowning in 1688-1689. He was survived by three children.