The park opened on April 15, 1976 and is named after Thomas F. McKinney, a businessman, race horse breeder and rancher, who owned and lived on the land in the mid-to-late 19th century.

The park is part of the El Camino Real de los Tejas National Historic Trail.

The calcium carbonate sediments deposited during this period lithified into the limestone rock underneath the park's soil and was exposed by erosion around the creek bed.

Aquatic reptiles swam in the sea as evidenced by a complete skeleton of a mosasaur found in the rocks of Onion Creek not far from the park.

The tribes found water in the creek and protection among the rock shelters created by the same undercutting action that formed the falls.

In Spanish Texas, McKinney Falls State Park was traversed as part of the El Camino Real de Los Tejas.

Heavy rains that started on October 30, 2013, pushed the water of Onion Creek to its highest level since the park opened.

Bald Cypress, sycamores and pecan trees line the banks of Onion Creek and live oak, ashe juniper, cedar elm and mesquite can be found on the drier uplands.

Other trees found in the park include Wafer Ash, Red Oak, Texas Persimmon, chinaberry and Mexican Plum.

[5] The problem was blamed on rain water runoff from the mostly urban area located upstream on Onion and Williamson Creeks.

This was due to the closure of the Williamson Creek Wastewater Treatment Plant in 1986, Austin's enactment of a comprehensive watershed ordinance and starting a citywide hazardous chemicals collection program.