At the 1986 World Judo Championships, Guy won a bronze medal in the under 61kg event. She is the only New Zealander to have won a medal at the World Judo Championships. In the same year, Guy came second at the judo demonstration event at the 1986 Commonwealth Games in Edinburgh, Scotland, losing in the final to Diane Bell. Guy also competed for New Zealand at the 1988 Summer Olympics in Seoul, South Korea. At the 1990 Commonwealth Games in Auckland, New Zealand, Guy-Halkyard came second in the under 61kg event, losing to Briton Diane Bell in the final.