While studying at the Pont-à-Mousson seminary in France, he decided to devote his life to the American missions, attaching himself to the Diocese of Cincinnati in Ohio.

[5] He received his consecration on April 26, 1857, from Purcell, with Archbishop John Henni and Bishop Joshua Young serving as co-consecrators.

[8][2] Juncker completed the first cathedral in the diocese in 1859, and founded two men's colleges, six girls' academies, a seminary, two hospitals, and one orphanage.

During his visit to Red Bud, Juncker heard confession from 1,000 Catholics and received a donation of land from a Protestant businessman for a new church.

When the Union Army opened a medical camp for wounded soldiers in Cairo, Illinois, he sent priests and nuns there to provide support.