The Kern Canyon fault, the longest fault in the southern Sierra Nevada, is an active structure and has been reactivated at discrete times over the past ∼100 m.y. in response to changing lithospheric stresses. After initiation as a Cretaceous transpressional structure, the Kern Canyon fault transitioned into a dextral strike-slip shear zone that remained active as it was exhumed into the brittle regime during regional Late Cretaceous uplift of the Sierra Nevada batholith. The Kern Canyon fault was reactivated during Miocene regional extension as part of a transfer zone between two differentially extending domains in the southern Sierra Nevada. Subsequent normal displacement along the fault began in Pliocene time. New evidence for fault activity, which continued into late Quaternary time, includes its current geomorphic expression as a series of meters-high, west-side-up scarps that crop out discontinuously along the fault's 130-km length. Relocated focal mechanisms of modern earthquakes confirm ongoing normal faulting, and geodetic measurements suggest that the Sierra Nevada is uplifting relative to the adjacent valleys. This evidence for recent activity overturns a long-held view that the Kern Canyon fault has been inactive for more than 3.5 m.y. Its reactivation indicates that deformation repeatedly localized along a preexisting crustal weakness, a Cretaceous shear zone. We propose that a system of interrelated normal faults, including the Kern Canyon fault, is responding to mantle lithosphere removal beneath the southern Sierra Nevada region. The location of the active Kern Canyon fault within the Sierra Nevada–Great Valley microplate indicates that deformation is occurring within the microplate.