Julien Lahaut was born in 1884 in Seraing, an industrial suburb of Liège in Wallonia, southern Belgium. A son of the working class, Julien was involved from an early age in the country’s great class struggles, such as the strikes to demand universal suffrage. His father, a boilermaker, was a pioneer in the local socialist section of the Iron City, as Seraing was nicknamed. At the age of fourteen, Julien left school, like all working-class children, and was hired by Cockerill, a huge steel company and at the time the flagship of Belgian industry. The whole economic life of the region revolved around this firm. However, working conditions there were precarious.

Lahaut joined the trade union movement at a young age. He was at the forefront of the metalworkers’ strike in 1902 — and was sacked in retaliation. He later found work in Val St-Lambert and cofounded the “Stand Up” union, the Liège-based forerunner of the later Metalworkers’ Union. He became a union official in 1908, following a strike at Val St-Lambert and a further dismissal. Another striker — his soon-to-be wife, Géraldine Noël — was dismissed at the same time for the same reason. Lahaut’s grassroots work enabled the Metalworkers’ Union to organize throughout the industrial area of Liège.