Dennis was born on September 28, 1940, in the Bronx, New York. He was the second of four children born to William and Theresa Collopy of McGraw Avenue. Shortly after high school, Dennis answered his country’s call and enlisted in the U.S. Army’s 82nd Airborne Division. Those years of service instilled in him the discipline and leadership that would mark every phase of his life.

In 1964, Dennis married the love of his life, Kathleen, and they built a life together in what became their home in Rockland County. Over the next 60 years, their bond stood as a testament to devotion, respect and partnership. Family was everything to Dennis—a loving husband, devoted father to daughters Julia and Lisa, and proud grandfather to his four grandsons, who were his world.

In 1967, Dennis joined the New York City Fire Department and served for 36 years, ultimately retiring as battalion chief of Battalion 11 on January 27, 2003. He found his second family in the fire department, first at Ladder 2/Engine 8, “Snoopy Truck,” then at Ladder 17/Engine 60, “the Green Berets,” and later at Ladder 26/Engine 58, known fondly as “The Fire Factory.” While in the fire department he received many awards, including Meritorious Awards, Service Rating A and B, and he was involved in two unit citations. He ended his career in Battalion 11 on 100th Street at Ladder 22/ Engine 76, a firehouse known as “Monumental Pride.” This was a perfect place for Dennis to end his career, as it sums up how he felt about the FDNY. Dennis was an effective leader—steady under pressure, fair in his expectations, and always leading by example, earning the respect and loyalty of every firefighter who served beside him.

Dennis’s service extended beyond the call of duty. In the years after the attack on the World Trade Center, he was among the dedicated citizens whose work there left an enduring legacy and whose health would later reflect the sacrifice. On January 30, 2025, Dennis lost his battle with a World Trade Center-related cancer; he left this world peacefully yet bravely, after a courageous battle.

Dennis was more than a firefighter and soldier. He was a man whose deep love for his family, lifelong pride in friendships spanning over eight decades, and unwavering readiness to answer the call defined every chapter of his life.