Hardwicke was born in London, England, and studied acting at RADA. He had made his film debut as a teenager, but remained very much in his father's shadow until becoming familiar to television audiences in the 1970s drama series, Colditz, in which he played Pat Grant, a character based on the real-life war hero, Pat Reid.
In 1986, Hardwicke took on the role of Dr. Watson from David Burke in a televison adaptation of the Sherlock Holmes stories, alongside Jeremy Brett. He became permanently associated with the role, also playing it on the West End stage with Brett in The Secret of Sherlock Holmes.