In 1916 he joined the weekly newspaper of the socialist party, Die Schmiede (The Forge).
In September 1916 he helped found the first socialist trade union, the Luxemburger Metallarbeiter-Verband.
On 30 May 1918 he was elected to the Chamber of Deputies for the canton Esch on the list of the Social Democrats.
From July 1919 onwards he was general secretary of the free trade unions and editor of their publication, Der Proletarier.
Pierre Krier was married to the trade unionist and women's rights activist Lily Becker.