Miguel Perez Jr. took his oath in a naturalization ceremony in Chicago, The Chicago Sun-Times reported.
"Here it is," he said, holding up his citizenship certificate for the cameras after being sworn in. "It's been a long journey, a long battle."
In 2008, he was accused of giving a laptop case of cocaine to an undercover police officer. He took a plea deal and served more than seven years in prison. Perez's citizenship application was tossed in 2010 following the conviction.
After serving his sentence, authorities turned him over to U.S. Immigration and Customs Enforcement (ICE), which detained him for two years before deporting him to Mexico.
Illinois Gov. J.B. Pritzker pardoned Perez in August, thereby erasing the conviction, in an effort to allow him to apply for citizenship.
"Miguel Perez should not have been deported,” the Democratic governor said at the time. “I recognize this pardon is not a perfect solution, but it is the most just action to take to allow a U.S. veteran the opportunity to be treated fairly by the country he served."
Immigration officials allowed Perez to return to the U.S. for two weeks last month to attend a Chicago immigration hearing.
On Friday, Perez said he plans to stay in Chicago to spend time with his family and advocate for "those that are left behind."
The Associated Press contributed to this report.