 Shirley's career personal best score of 8036 points was achieved at the 1988 Olympic Games in Seoul, where he finished 15th behind gold medalist Christian Schenk of East Germany.Shirley continued to compete for Australia throughout 1989 but thereafter stated his intention to return to his birth nation of England and compete for Great Britain.His first significant performance as a British athlete was in June 1993 when he won the decathlon at the Azusa Pacific University track meet with a score of 7944 points.