In January 1905, Mangin requested an introduction to discuss Tonkin with the nationalist activist and former minister, Godefroy Cavaignac. This was arranged through Marie Georges Humbert, the professor of mathematics at the École Polytechnique, and married to his deceased wife's sister, Marie Jagerschmidt. She gave private lessons to Cavaignac's daughter Antoinette. Following the interview, Cavaignac regularly invited Mangin to dine at his house where his 25-year-old daughter Antoinette was living. Whilst Mangin was impressed by Antoinette's intelligence, he in turn impressed her with accounts of his exploits in the colonies. In June 1905, Mangin proposed and the couple were married on July 31, 1905.