Killanin was born in London, member of one of the fourteen families making up the Tribes of Galway. He was educated at Eton and later Magdalene College (Cambridge). In the late 1930s, he began his career as a journalist, which was interrupted by World War II, in which he served as a volunteer.

In 1950, he became the head of the Olympic Council of Ireland, and became his country's representative in the IOC in 1952. He climbed up to senior vice-president in 1968, and was elected president of the IOC in 1972, following the Munich Olympics, which were overshadowed by the Munich Massacre.

During his presidency, the Olympic movement experienced a difficult period, dealing with the boycotts of the 1976 and 1980 Summer Olympics. He resigned after the Moscow Olympics in 1980, and his position was taken over by Juan Antonio Samaranch.

Killanin was also a director of many companies and dabbled in the film industry, collaborating with his lifelong friend, John Ford, on "The Quiet Man".

He died at his home in Dublin aged 84 and, following a funeral service in Spiddal, County Galway, he was buried in the family vault in the New Cemetery, Galway.