Son of the late John W. and Louise (Cook) Slagle, John was a 1947 graduate of Somerset High School. He enlisted in the U.S. Army during the Korean War, and served as a military intelligence officer stationed in Germany. After marrying the love of his life, Audrey (Peck), in 1954, he completed his pursuit of a B.A .in Political Science from Duquesne University, Pittsburgh, and went on to earn another degree from the American Institute of Foreign Trade in Scottsdale, Arizona. For 30 years, he worked for the U.S. Bureau of Customs as a textiles specialist supporting the Port of Philadelphia, also serving for several years as president of the Philadelphia Federal Employees Credit Union. Following his retirement from Customs, he worked for the Wolf D. Barth Company as a Director handling international trade.

John's life was guided by his deep faith. A devout Roman Catholic, he was an active member of St. Rose of Lima Parish, Eddystone, for 61 years, serving as Lector Coordinator and lector, as well as Extraordinary Minister of Communion. He was a daily communicant and a model of how to live the Catholic faith. He will be especially missed by attendees of the 7:30 AM Sunday Mass.

John and Audrey traveled extensively throughout the United States and Europe, as well as to New Zealand, China, Ireland, Canada, and their favorite Caribbean destination, St. Maarten. John also enjoyed gardening, spending time with his friends, and interacting with his children.