Gray was sworn in at 11:57 a.m. Sunday. His staff tweeted his remarks throughout the ceremony.
"Today we begin a new chapter," he told the assembled crowd. "A chapter not written by a single author, but with the pens of 600,000 residents."
Gray defeated incumbent mayor Adrian Fenty to take the office. The two men embraced on stage during the ceremony.
During the swearing-in ceremony, Gray also sounded an ambitious note on District voting rights.
"We cannot rest," he said, "until we achieve true self-determination and become our nation's 51st state."