He joined the British Indian Army as a teenager and then the Royal Engineers, reaching the rank of lieutenant.

"[1] In 1975, Astles joined Amin's service, becoming the head of the anti-corruption squad and advising the president on British affairs, while running a pineapple farm.

[1] Following the Uganda–Tanzania War, Astles fled Uganda by crossing Lake Victoria in a canoe to Kenya on 10 April 1979.

[4] While detained in Kenya, Astles tried to jump from a third-story window in an apparent suicide attempt in Nairobi.

[6][4] At the time Kenyan newspapers linked the charges to the death of Bruce McKenzie, one of Kenya's former ministers, in an aircraft explosion.

Astles was charged with murder, armed robbery, theft, shopbreaking, and stealing from a private house.

Astles said he had lived in constant fear while in Amin's administration, and that he had remained in his position because he thought he could do some good for Uganda.

[4] Astles was acquitted of the charges, but he remained in Luzira Prison for another four years under a detention order because he was considered a threat to state security.

A year later, after they had divorced, Astles married an aristocratic member of the Buganda kingdom, Mary Senkatuka, and they later adopted two children.

[13] After returning to Britain, he dedicated his life to campaigning against superpower interference in African political and economic affairs.

The fictional character of Nicholas Garrigan in the book and film The Last King of Scotland was, according to author Giles Foden, loosely based on some events in Astles' life.