Born in Toulouse, Séguy's father was a communist and trade unionist, and Pierre Semard was a family friend.

He joined a trade union, part of the Railway Workers' Federation, which in turn was an affiliate of the CGT.

He soon rose to prominence in the union, being appointed to its secretariat in 1949, and continuing to serve when transferred to Paris, and then Montreuil.

From 1970, he served on the executive of the World Federation of Trade Unions, but he became increasingly critical of its policy in Eastern Europe, and resigned in 1977.

[1][2][3] Séguy retired in 1982, but continued to sit on the CGT executive, and he also set up the Institute for Social History, becoming its president.