Clarion County was created on March 11, 1839, from parts of Venango and Armstrong Counties and named for the Clarion River. Clarion, the county seat, was incorporated as a borough on April 6, 1841.

Clarion County is located in the northwestern part of the state, with the Allegheny River forming the largest part of the southern boundary. Clarion County has a forest area of approximately 160,000 acres, the largest portion of which is Cook Forest. Cook Forest has the largest stand of virgin white pines east of the Mississippi River and has been designated as a National Natural Landmark. Much of the area exists almost as it was in the days of William Penn when it was known as "The Black Forest" or "Penn's Woods." It was this pristine beauty that drove Cecil B. DeMille to use Cook Forest as the backdrop for Paramount Pictures' 1946 film 'The Unconquered' starring Gary Cooper and Paulette Goddard.

The Clarion and Allegheny Rivers provide many recreational and scenic areas within the county. The state parks and state game lands provide facilities for boating, camping, fishing, hunting, hiking, mountain biking, and swimming.

The main function of Clarion, the County Seat, is to provide a base for the recreation available in the region. Clarion borders on the Piney Dam Reservoir, a deep, wide and six-mile long body of water, suitable for everything from fishing to water skiing. Upstream, as the Clarion River narrows back into a shallow stream, it runs through Cook Forest State Park.

In October, Clarion is the site of the Autumn Leaf Festival, a celebration that draws 500,000 people to the community, and has been held annually for more than 40 years. Craft shows, antique auto show, carnival, talent pageant, and food concessions all make the festival northeast Pennsylvania's premier rural festival.