McCarthy died in his sleep at an assisted living facility in Washington's Georgetown neighborhood, the New York Times reported.
The former college professor represented Minnesota in the House for eight years and 12 in the Senate. He was best known for his 1968 campaign for the Democratic presidential nomination, when battalions of anti-war college students turned out to work for him.
McCarthy's strong showing in the New Hampshire primary convinced Johnson to withdraw from the race. In an election season that included Robert Kennedy's assassination, Vice President Hubert Humphrey won the nomination and went on to lose to Richard Nixon in a close election where McCarthy's presence as an independent candidate may have tipped the balance.
"We do not need presidents who are bigger than the country, but rather ones who speak for it and support it," McCarthy said during his campaign against Johnson.
In later years, McCarthy lived on a farm in Virginia's Blue Ridge Mountains, writing and sometimes teaching college.