Dennis is notable for a number of reasons. He introduced new antiseptic methods in the hospitals' surgical wards. With his friend Dr. W.H. Welch, he helped organized the Carnegie Laboratory. He served as professor of surgery at Bellevue Medical School and was a popular instructor; he was professor emeritus of clinical surgery at New York Cornell Medical School and a consulting surgeon at several hospitals; he was the oldest surviving member of the American Surgical Association and served as vice president of the organization in 1887.
Dennis built the lodge in 1909 and spent what free time he had developing the estate in the summer months. In addition to gardens and other horticulture, he also operated a horse stable. As a member of the Norfolk elite, he was friends with many influential people, including President William Howard Taft and Andrew Carnegie, whom he hosted at the lodge. Dennis apparently died suddenly in 1934 in New York City, not having shown signs of illness or stress.