Born in Ostend, Dekeyzer became an English teacher, and the unpaid secretary of the Socialist Union of Education Workers of West Flanders.
In 1935, he began working full-time for the Belgian Union of Transport Workers (BTB), as deputy secretary of its West Flanders district.
He fled to the United Kingdom early in World War II, and worked organising Belgian sailors there.
[1] Dekeyzer returned to Belgium at the end of the war, and was elected as secretary of the General Federation of Belgian Labour (ABVV).
[1][2] Dekeyzer retired from his trade union and political posts in 1971, but remained on the boards of various organisations for the next decade.