Born in Billericay, Denmark began his international career by winning a bronze medal in the 3000 metres at the 1991 World Indoor Championships.

A year later, he achieved his lifetime best in the 5000m with 13:10.24 at the Rome Grand Prix on 9 June, a time that still ranks him sixth on the UK all-time list (as of 2017).

In 1994, he won a silver medal in the 5000m at the European Championships behind Olympic champion Dieter Baumann, before going on to win the 5000m title at the Commonwealth Games a month later.

Denmark went on to compete at three more World Championships (1995–99), and in the 10,000m at the 2000 Sydney Olympics, without reaching the finals.

Denmark attended Furtherwick Park School,[1] on Canvey Island, and represented Basildon Athletic Club.