During the Second World War (1939–1945) he was commissioned as a second lieutenant, in 1941, into the 4th Battalion, Grenadier Guards and served with it throughout the North-West Europe Campaign of 1944–45.
In the late 1940s Hawkesworth began his film career as an assistant to art director Vincent Korda working with him on The Fallen Idol (1948).
In the mid-1960s, he began to work for television, and he wrote the scripts for programmes including The Hidden Truth (1964), The Short Stories of Conan Doyle (1967) and The Gold Robbers (1969).
During the 1980s, he produced many television programmes including By the Sword Divided (1983), The Adventures of Sherlock Holmes (1984) and Oscar (1985).
In his biography of the television producer Verity Lambert, Richard Marson describes Hyacinth as "fiercely snobbish", and as someone who "answered to the unlikely nickname 'Pussy'".