Peniakoff began an engineering degree at the Free University of Brussels at the age of 15 before his studies were interrupted by the German invasion of Belgium in August 1914.

He initially had conscientious objections to participation in World War I, but by his fourth term at Cambridge his views had altered, and he went to France to volunteer as a gunner in the French artillery.

Peniakoff was commissioned as a second lieutenant on the British Army General List on 4 October 1940, serving in the Libyan Arab Force.

He was given the nickname Popski, after a Daily Mirror cartoon character,[4] by Captain Bill Kennedy Shaw (the LRDG's Intelligence Officer) because his signallers had problems with the name Peniakoff.

Peniakoff became the British-Russian liaison officer in Vienna before demobilisation, naturalisation and achieving fame as a British writer and broadcaster.