One of the most important medical technologies developed in the post-World War II era was the heart-lung machine, which made possible a vast expansion in the surgical treatment of heart disease. Surgeon and medical educator Clarence Dennis was a leading pioneer in this field, inventing one of the earliest heart-lung bypass machines, and attempting the world's first open-heart operation supported with such a device.

Dennis was born in St. Paul, Minnesota on June 16, 1909, to Warren Dennis, a noted surgeon, and Clara Van Orman Dennis. He was the second of their six children. Serious and hardworking, Clarence was an outstanding student, and graduated fourth in his high school class of 333. He had hoped to become an engineer--during several summer jobs in construction work he had discovered that he enjoyed working with his hands and had a talent for building things. His mother, however, decided that her second son should become a doctor like his father (who had died from pneumonia in 1923) and she would not provide financial support for any other college degree. Accordingly, Dennis attended Harvard College (where he was elected to Phi Beta Kappa and graduated magna cum laude in 1931) and then went on to the Johns Hopkins School of Medicine. After receiving his MD in 1935, he returned to Minnesota to do an internship and surgical residency with Owen H. Wangensteen at the University of Minnesota Hospital. Wangensteen's program required substantial laboratory research leading to a PhD; Dennis soon discovered that he enjoyed the lab work as much as the operating room. He earned a MS in physiology under Maurice Visscher's direction in 1938, and a PhD in surgery in 1940. He then joined the faculty of the department of surgery at the University of Minnesota, becoming full professor in 1947.

During his early career, Dennis's surgical research focus was on intestinal conditions: intestinal obstruction, appendicitis, ulcerative colitis, etc. He worked with Wangensteen and with Visscher to elucidate the origins of appendicitis, and devised a number of innovative surgical techniques and procedures for other conditions. Soon after World War II ended, however, his two mentors suggested a new research project: developing a pump-oxygenator device to maintain blood circulation long enough to allow open-heart operations. Surgeons were just beginning to attempt repairs to the heart, an organ long believed to be "off limits." Their progress, however, was severely limited by the difficulty of stopping the heart long enough to operate on it. The idea captivated Dennis, who had earlier invented a number of devices to aid his surgical research. He interviewed everyone he could about work in this area, and soon met Dr. John H. Gibbon, who had been working on a pump-oxygenator since about 1934. The two became friends and exchanged ideas and information about their parallel projects as they struggled with the many technical challenges of mechanical heart-lung devices. In April 1951, after numerous trials with dogs, Dennis and his team became the first to use a pump-oxygenator to perform open heart surgery on a human patient. Though the machine performed very well, the surgeons were unable to save the six-year-old patient, because her heart defect turned out to be much more extensive than the atrial septal defect they had diagnosed. Dennis's second attempted open heart operation, several weeks later, also failed, when a technician's error caused a fatal air embolism. Dennis continued to improve his heart-lung machine, and in 1955 completed his first successful cardiac operation with it, two years after John Gibbon's first success in 1953.

Dennis moved to New York in mid-1951 to chair the department of surgery at the State University of New York's Downstate Medical Center in Brooklyn (formerly the Long Island College of Medicine). As chair, he helped build the department and develop residency and research programs during Downstate's rapid expansion in the 1950s. With the members of his Minnesota team who moved with him, he also continued his surgical research, perfecting his heart-lung machine. He went on to use cardiopulmonary bypass to sustain patients suffering cardiac shock following heart attacks.

After twenty years at SUNY, Dennis retired in 1972, and moved to the National Institutes of Health in Bethesda, MD, to direct the Division of Technological Applications (formerly the Medical Devices Application Branch, which in turn grew from the Artificial Heart Program) at the National Heart and Lung Institute. The DTA was responsible for coordinating federally funded research and development in cardiovascular and pulmonary devices, including circulatory assistance devices and total artificial hearts. The artificial heart initiative, begun in 1963, envisioned combining biomedical research with systems engineering to produce a totally implantable mechanical heart for human patients by 1970. This proved considerably more difficult than expected, and part of Dennis's task was to analyze the management of engineering contracts and medical research grants for the project, and to make changes if necessary. His efforts to streamline DTA operations were often resisted by the staff and ignored by his superiors. The DTA was disbanded in 1973 due to federal budget cuts, and Dennis stayed on for another year as Special Assistant for Technology in the NHLI director's office. He resigned in 1974, convinced that his expertise was being wasted.

In 1975, Dennis returned to academic medicine, joining the surgical faculty at the SUNY medical school at Stony Brook. There, he continued to teach and carried out studies on wound healing and gastrointestinal diseases. He retired again in 1988 and moved back to Minnesota. The retirement was short-lived, however; in 1991, at the age of 82, he was recruited by the University of Minnesota to direct its Cancer Detection Center. The center had been established by Dennis's mentor, Owen Wangensteen, in 1948, to encourage early detection of cancer through regular screening. When Dennis took over, the center was suffering from declining client numbers, inefficient coordination of diagnostic services, and a dilapidated clinical space. He worked valiantly for several years to resolve these problems and keep the CDC operating, but severe budget shortages in the department of surgery forced the center to close in 1996. At 87, coping with progressive macular degeneration, Dennis felt ready to retire for the third and final time.

Dennis published nearly 200 articles and book chapters, many of them on heart-lung machines and cardiac assistance devices, but also many on surgical treatment of gastrointestinal problems as well as other topics. His accomplishments were recognized by many honors and awards, including the Modern Medicine Distinguished Achievement Award in 1973; the American Society of Extracorporeal Technicians first annual John H. Gibbon Award in 1974; the Association for the Advancement of Medical Instrumentation's Laufman-Greatbatch Prize for "Outstanding Pioneer in the Development and Application of Cardiovascular Surgical Development and Application of Cardiovascular Surgical Devices" in 1984; and an honorary Doctor of Science degree from SUNY Health Sciences Center, Brooklyn, in 1988.

Dennis was an active member of many professional societies for much of his career, including the American Surgical Association, the American College of Surgeons, the American Society for Artificial Internal Organs, the International Society of Surgery, and the National Society for Medical Research. Through such groups he contributed to efforts ranging from setting standards for surgical education and ethics, to shaping amendments to the federal Food, Drug, and Cosmetic Act (to regulate the growing number of biomedical devices on the market), to defending the use of animals in medical research.

Dennis met Eleanor Smith, a friend of his sister Jane, when he returned to Minnesota after medical school. They married in 1939, and had a daughter and three sons, two of whom became surgeons. The couple divorced in 1976. Dennis married Mary Mott in 1977.

Dennis died in St. Paul, Minnesota on July 11, 2005, at the age of 96, from complications of dementia.