It was a long-awaited encounter symbolizing Northern Ireland's progress in achieving reconciliation after decades of violence.
The monarch and Martin McGuinness met inside Belfast's riverside Lyric Theatre and shook hands privately, but the two shook hands again a half-hour later for the public, documenting a moment that would have been inconceivable back in the days when IRA leaders were plotting to kill the British royal family.
Underlying the sensitivity of the occasion, no live footage or sound was permitted to be broadcast, and outside, police shut down all roads surrounding the theater and told residents to stay inside.
Many people believe McGuinness was a top IRA member at the time of a notorious attack known as the Remembrance Day Bombing. McGuinness says he wasn't in the group at the time. He's now a senior member of Northern Ireland's unity government.
The 86-year-old head of state didn't speak but kept smiling as she shared a stage with a man linked to the killing of her cousin Lord Louis Mountbatten.
While the queen showed no sign of unease, the same couldn't be said of her 91-year-old husband, Prince Philip. At one point, as McGuinness sought to engage him in conversation, Philip quickly shot ahead to his wife's side. And at the farewell, McGuinness got only a quick, silent handshake from Philip, who was Mountbatten's nephew.
The queen was in Belfast officially to celebrate her 60th year on the throne. On Tuesday, she visited the site of the Remembrance Day Bombing and made her first visit as queen to a North Ireland catholic church.
The Associated Press contributed to this report.