John Anthony Ranulf was born in Bath to Reginald Sparshott Thatcher and his wife, Ruth Trethowan. He had a sister.

At the time, his father, the son of a brewer of Midsomer Norton, was Director of Music at Charterhouse School. He had fought in the First War when he won the Military Cross. He went on to be Director of Music at Harrow from 1928 until 1936. He was Deputy Director of Music at the BBC and Principal of the Royal Academy of Music from 1949 and died in 1957 having been knighted.

JAR was educated at Charterhouse and Matriculated in 1938 being awarded a Holford Scholarship. He was up for only one year.

He was gazetted as a Second Lieutenant with an emergency commission in the Indian Army on 27 July 1940 and sailed for Bombay from Liverpool on 22 October on the Britannia, to join the 16th Punjab Regiment. 

The Regiment fought in India and moved to Malaysia to defend Singapore. In February 1942, the 2nd Battalion was captured by the Japanese at the surrender of Singapore.

He was killed on 20 January having been Mentioned in Despatches.

He is commemorated on the Singapore Memorial Column 263.