Mackerras was born in Schenectady, New York, and was brought up in Sydney, Australia. He went to the New South Wales Conservatory to study oboe, piano and composing. He became an oboe player with the Sydney Symphony Orchestra during World War II, and was made the principal oboist in 1946. He conducted the orchestra at the opening of the Sydney Opera House in 1973. Five years later he was the first Australian conductor to be the orchestra's principal conductor.