The decision Thursday by immigration officials means Miguel Perez Jr. can be deported to Mexico at any time.
The 39-year-old Perez petitioned immigration officials to be granted citizenship retroactive to when he joined the military in 2001.
Perez served two tours in Afghanistan. He says he mistakenly thought he became a U.S. citizen when he took an oath to protect the nation.
Perez was taken into custody by Immigration and Customs Enforcement officers after he served half of a 15-year prison sentence for a nonviolent drug charge. He is being held in a Wisconsin detention center where he awaits deportation.