PHILADELPHIA—A Philadelphia jury delivered a sharp rebuke of the Catholic Church's handling of allegations of sexual abuse by its priests, convicting for the first time a senior church official of not taking steps that could have prevented further attacks.

Msgr. William Lynn, who served as secretary for clergy in the Archdiocese of Philadelphia from 1992 to 2004, was found guilty on Friday of one count of child endangerment for allowing a priest to take a new assignment involving contact with children even after learning of...