Lee Reinhart, 38, was kicked out of the Coast Guard in 2002 for being gay, the Chicago Tribune reported.
Reinhart enlisted in the Navy Reserve in Chicago. U.S. Rep. Mike Quigley, a North Side Chicago Democrat who voted to repeal "don't ask, don't tell," administered the oath to Reinhart during a ceremony.
Reinhart said he has a "desire to be able to give back to my country and serve my country, and the honor and pride that comes with that."
"I just feel I have a place there," Reinhart said of the military. "I'm glad I took the path that I did."
Reinhart first enlisted in the Navy in 1995 and mostly kept his sexual orientation a secret, telling only close friends. He finished his commitment in 1999
"I never had a negative experience the rest of my time in the Navy," Reinhart said. "I tell people that I was on the right ship at the right time. And that says a lot for the crew of that ship."
He then enlisted in the Coast Guard a few months after the Sept. 11, 2001, terrorist attack. Four months after joining, he was discharged after word spread that he had gone to a gay bar while his ship was at port.
"That's not what the military teaches," he said. "That's not the core of the military, and that was a problem with the policy."
Reinhart then spent years in Ohio lobbying for "don't ask, don't tell" to be repealed.
"There are some people in this world that talk about doing a lot of good, and Lee is one those people who went out and actually made something happen," said Michael Landry, a chief warrant officer in the Navy who served with Reinhart. "He worked tirelessly after getting out of the military to get 'don't ask, don't tell' repealed. He did a lot of good for a lot of other people, and I want to be there for him."