Downie represented Great Britain at the 1973 World Aquatics Championships in Belgrade, Yugoslavia where he broke the Scottish record for the 200-metre freestyle. The next year he represented Scotland at the 1974 British Commonwealth Games in Christchurch, New Zealand. In 1975 he swam for Scotland at the 8-nations tournament in Prague, Czechoslovakia. and won a silver medal at the 1975 World Championships in Cali, Colombia as part of the British 4x200-metre freestyle relay with Alan McClatchey, Gary Jameson, and Brian Brinkley. At the same championships he won a bronze medal as part of the British 4x100-metre medley relay with David Wilkie, James Carter and Brian Brinkley. Apart from winning his bronze medal, he was sixth in the 200-metre freestyle at the 1976 Montreal Olympics when he set a British record that stood for six years. He swam in the 1977 European Aquatics Championships in Jönköping, Sweden, and represented Scotland in the 1978 Commonwealth Games in Edmonton, Alberta, Canada. Downie held both the Scottish 200-metre and 400-metre freestyle records for 10 years from 1972 to 1982.