Harmsworth was a medallist at the 1987 European Athletics Indoor Championships, coming third in the 400 metres, and finished in fifth place at the 1987 IAAF World Indoor Championships, where he was number one European. Nationally, he was the British indoor champion that same year. He was the 1980 British youth champion in 1980. He also set British and Commonwealth indoor records in the 4 × 400 m in 1987, and outdoors in the 4 × 200 m in 1988.