O'Callaghan was transferred to the U.S. Army in 1952 to see combat and lost part of his left leg after being hit by a mortar round during a battle in the Korean War.

O'Callaghan resumed his college studies at the University of Idaho in Moscow, where he was a member of Tau Kappa Epsilon fraternity,[3] and completed his bachelor's and master's degree in education in 1956,[4] then became a high school teacher and boxing coach in Nevada.

[5] O'Callaghan's political career began in 1963, when Governor Grant Sawyer appointed him to head the state's new department of health and welfare.

In 1970, he received the Democratic gubernatorial nomination and won a surprising victory in the general election over his Republican opponent, Edward Fike.

Mike O'Callaghan died on March 5, 2004, of a heart attack at the age of 74, after collapsing during the morning mass hours at the Saint Viator Catholic Church in Las Vegas, Nevada.

[6][7] His widow Carolyn, a native of Twin Falls, Idaho, died seven months later on October 7, 2004, of complications from cardiac surgery, at the age of 68.