Bentine was born in Watford, Hertfordshire, to a Peruvian father, Adam Bentin, and a British mother, Florence Dawkins,and grew up in Folkestone, Kent. He was educated at Eton College. With the help of speech trainer, Harry Burgess, he learned to manage a stammer and subsequently developed an interest in amateur theatricals, along with the Tomlinson family, including the young David Tomlinson. He spoke fluent Spanish and French.
His father was an early aeronautical engineer for the Sopwith Aviation Company during and after World War I and invented a tension meter for setting the tension on aircraft rigging wires.
In World War II, he volunteered for all services when the war broke out (the RAF was his first choice owing to the influence of his father's experience), but was initially rejected because of his father's nationality.
He started his acting career in 1940, in a touring company in Cardiff playing a juvenile lead in Sweet Lavender. He went on to join Robert Atkins' Shakespearean company in Regent's Park, London, until he was called up for service in the RAF. He was appearing in a Shakespearean play in doublet and hose in the open-air theatre in London's Hyde Park when two RAF Police NCO's marched on stage and arrested him for desertion. Unknown to him, an RAF conscription notice had been following him for a month as his company toured.
Once in the RAF he went through flying training. He was the penultimate man going through a medical line receiving inoculations for typhoid with the other flight candidates in his class (they were going to Canada to receive new aircraft) when the vaccine ran out. They refilled the bottle to inoculate him and the other man as well. By mistake they loaded a pure culture of typhoid. The other man died immediately, and Bentine was in a coma for six weeks. When he regained consciousness his eyesight was ruined, leaving him myopic for the rest of his life. Since he was no longer physically qualified for flying, he was transferred to RAF Intelligence and seconded to MI9, a unit that was dedicated to supporting resistance movements and helping prisoners escape. His immediate superior was the Colditz escapee Airey Neave.
At the end of the war, he took part in the liberation of Bergen-Belsen concentration camp. He said about this experience:
Millions of words have been written about these horror camps, many of them by inmates of those unbelievable places. I’ve tried, without success, to describe it from my own point of view, but the words won’t come. To me Belsen was the ultimate blasphemy. (The Reluctant Jester, Chapter 17.)
