Of Finnish and American parentage, he spoke nine languages fluently; international issues were to absorb much of his energy, both as an official of the League of Nations between the wars, and as a Labour member of the House of Commons in the post-War period.
Zilliacus talade nio språk flytande och internationella frågor skulle absorbera en stor del av hans energi, både som en tjänsteman vid Nationernas förbund mellan krigen och som en Labourmedlem i Underhuset under efterkrigstiden.