Terry Brown, 38, was charged with third-degree aggravated battery, for biting off Patrick Pinto's left thumb at the first knuckle during a Sept. 26 fight, Undersheriff Mark McCloskey said.
The severed thumb was found in a field near Dino's Hidaway, but too much time had passed to allow for it to be reattached, police said.
San Juan County Sheriff's deputies responded to the Farmington bar after an employee called 911.
The bar owner declined to comment.
Pinto, of Farmington, showed up at Dino's and advised security that he was in a fight and the person he was fighting bit off his thumb, police said.
Pinto was admitted to San Juan Regional Medical Center.
Pinto met Brown on the bus from Albuquerque to Farmington after attending the state fair.
The two pooled their money together to buy beer, police said.
Pinto and Brown were drinking beer in the hills near Dino's Hidaway on County Road 5589 when the fight occurred.
Brown was becoming intoxicated and when Pinto, who recently was released from prison, tried to leave, Brown attacked him, police said.
Brown is missing one front tooth, which is not a result of the biting, police said.