New excavations along the Panama Canal have yielded a growing Early Miocene assemblage of mammals referred to as the Centenario Fauna. Despite the area’s proximity to South America, the mammals of the Centenario Fauna have entirely North American affinities. The Centenario Fauna is distributed throughout a ∼115-m stratigraphic interval encompassing the uppermost Culebra and Cucaracha Formations within the Panama Canal basin. Previously published ages constrain the age of the lower limit of the Centenario Fauna to no younger than ∼19 Ma, but the upper limit has remained problematical. A fresh exposure of the Cucaracha tuff, a prominent marker horizon within our measured sections, has yielded two new radioisotopic determinations: (1) an 40Ar/39Ar age of 18.96 ± 0.90 Ma and (2) a U-Pb zircon age of 18.81 ± 0.30 Ma. In addition, magnetostratigraphic data indicate that the Centenario Fauna occurs within chron C5Er, from 18.78 to 19.05 Ma on the geomagnetic polarity timescale of Gee and Kent. These correlations further confirm the calibration of the latest Arikareean (Ar4) to early Hemingfordian (He1) transition in Nebraska, at the base of chron C5Er, at about 19.05 Ma. The Centenario Fauna occurs at the beginning of the Hemingfordian North American Land Mammal Age, i.e., He1. A broad faunal province existed during the early Hemingfordian that can be recognized across a north-south range of 5000 km throughout North America, with the southernmost limits defined by the Centenario Fauna of Panama.