Donlin first became a member of the Knights of Columbus at the Father Otto Weber Council 2369,
Park Falls, Wisconsin, in 1965, and served two terms as grand knight of that order. He was an active member of the Catholic fraternal organization for the remainder of his life, and, from the time of his judicial resignation, worked as a national officer of the organization. As supreme advocate—the equivalent of
general counsel—Judge Donlin headed up the legal department at the Connecticut headquarters of the Knights of Columbus, managing legal affairs throughout the United States, Canada, Mexico, the Philippines, and Latin America. In 1985, he was appointed a knight of the
Order of St. Gregory the Great by
Pope John Paul II. As a representative of the Knights of Columbus, he was active in the affairs of the
National Fraternal Congress of America, and was elected its president in 1990. He worked for the organization until his death in 1996, at age 58. ==Personal life and family==