A member of Grant's staff, he'd returned to Washington City immediately after the surrender ceremony at Appomattox Courthouse and was only a few blocks away from Ford's Theater when his father was shot. Summoned to his side, he was present when his father died.
Sixteen years later, he was the Secretary of War for President Garfield and was standing beside him when he was shot.
Twenty years later, Robert Lincoln was retired from public life but accepted an invitation from President McKinley to attend the Pan American Expo in Buffalo and was about fifty feet from the President when he was shot.
After that, Lincoln turned down all invitations that would put him in proximity to the President. The only time he accepted such an invitation was in 1922, for the dedication of the Lincoln Memorial. Barely a year later, President Harding suffered a heart attack and died in office.