Born in Gelnhausen, Brey completed an apprenticeship as a shoemaker, and joined the Social Democratic Party of Germany (SPD) in 1885.
In 1890, he was a leading founder of the Union of Factory, Agricultural and Commercial Support Workers in Germany (FAV), a general union, and became its president.
He served until it was replaced by the Weimar National Assembly, and then in the Reichstag again.
In 1925, he was elected as president of the International Federation of General Factory Workers.
[1][2][3] In 1931, Brey retired as president of the FAV, and he left his political roles in 1932.