New York Senator Charles Schumer helped make it happen on the fourth try.
Mohammed was part of a Brooklyn swearing-in ceremony on Friday for about 255 new citizens.
"I'm relieved," the 22-year-old Mohammed said. "Now I can actually move on."
He wants to go to college and become a police officer.
Mohammed said he already felt like a citizen, because in the Army, "You wear that flag on your shoulder every day."
