Citrine joined the Electrical Trades Union (ETU) in 1911 and within a few years was the leading activist for that union in Merseyside, leading a national electrician's dispute there in 1913. He was elected as the union's first full-time District Secretary in 1914 (the year he married his wife and life-time companion, Doris), a post he served in throughout World War 1 and until 1920, gaining much experience negotiating with major employers all round Birkenhead docks as well as with electrical contractors in the area. He became secretary of the regional Federation of Engineering and Shipbuilding Trades (FEST) in 1919 and was elected as Assistant General Secretary of the ETU in 1920 at their headquarters in Manchester. In this role he transformed the union's finances with administrative changes which secured their income, creating his reputation for these unusual union skills.