Guérin temporarily halted his studies and joined the Vaucluse mobiles, seeing action at the Loire and during the Eastern campaigns of the war.

Following the end of the war in 1871, Guérin resumed his studies and graduated that same year with a doctorate in law.

He married Césarée Madeleine Marie Thérèse Pascal (1859-1936) on 20 October 1880 in Aubagne.

Guérin later served as Mayor of Carpentras from October 1881 to December 1886.

[3] It was during Guérin's second mandate that the Dreyfus affair occurred shortly before the end of his term in January 1895 Guérin ran for re-election to the Senate in 1900 and won his bid on 23 January 1900.