The son of Aileen (née Robertson) and William Slim (later 1st Viscount Slim), he was born in Quetta in British India and was educated at Prince of Wales Royal Indian Military College in Dehradun.

In 1944, Slim joined the 6th Gurkha Rifles of the British Indian Army and was later transferred to the Argyll and Sutherland Highlanders in 1948.

He was further director of Trailfinders travel company and Trustee of the Royal Commonwealth Ex-Services League (RCEL).

He was also Patron of Prospect Burma, a London-based charity that offers higher education scholarships to Burmese students as well as the Graham Layton Trust, a British charity which helps to raise money for eyecare in Pakistan.

Slim was married to cordon bleu chef Elisabeth Spinney from 1958 until her death in 2018.