Although Fonst was born in Cuba, he spent most of his youth in France, where he received his fencing education, and aged just 16 years old he entered the 1900 Summer Olympics which were being held in Paris. He entered the Men's épée event, which put him up against another 101 fencers from 10 other countries, in his first round group he came out top against the five fencers from France, so advanced into the next round, in the next round and again in a group of six fencers Fonst any managed to finish in third place but was still good enough to advance to the semi-finals, and again he would finish third in the group and so qualifying for the final.