With the outbreak of the First World War, Mordvinov joined the French Foreign Legion as a soldier. At the end of 1914, he returned to Russia; in August 1915, he was sent to France with an assignment from the Main Military-Technical Department of the Russian Army to promote ‘the production in Russia of special steel for forging shafts for aircraft engines and the development of a type of 300-horsepower engine’. He remained in Paris, where he died; buried in Passy Cemetery.