Born in Aylesbury, Buckinghamshire, he was the son of Thomas Stranks, a labourer and his wife Lydia, née Ironmonger. The family moved to Lichfield in Staffordshire where Sidney completed an apprenticeship. He joined the Operative Society of Masons, Quarrymen and Allied Trades of England in 1888 and became involved in the campaign for independent labour movement representation on political bodies. He moved to Lincolnshire, Manchester and then Croydon. At the 1906 general election, he was selected to stand for Croydon by the Labour Representation Committee, coming in a poor third place.