Immediately on completion of his PhD, McKellar was appointed as lecturer at the University of Sydney. In 1965, McKellar was invited by J. Robert Oppenheimer to become a member of the Institute for Advanced Study, Princeton, USA, which he undertook from 1966 to 1968. He then returned to University of Sydney. In 1972, at the age of 30, McKellar was appointed as the Professor of Theoretical Physics at Melbourne University. He retired from this role in 2007.