He made major contributions to Catholic social teaching which helped lay the groundwork for Leo XIII's encyclical Rerum novarum.

He was the eldest of six children of Jacques and Pernette (née Mégard) Mermillod, both born of farming families of a neighboring village of Bardonnex.

Gaspard attended the minor Seminary of Saint-Louis du Mont (1837-1841) at Chambéry, France and studied philosophy and theology at the Jesuit Collège Saint-Michel (1841-1847) at Fribourg, Switzerland.

He was especially active for Catholic education, founding with Louis Brisson and Léonie Aviat the Oblate Sisters of St. Francis de Sales at Troyes, for the protection of poor working girls.

[5] Normal relations resumed only when Leo XIII elevated Mermillod to Cardinal-Priest of Santi Nereo ed Achilleo on 23 June 1890.

[1] In March 1891, Mermillod resigned the pastoral government of the Diocese of Lausanne and Geneva, and Joseph Déruaz was named his successor.

His Lettres à un Protestant sur l'autorité de l'église et le schisme (Paris, 1860) made a great impression.