Dennis Franz Schlachta, born in Maywood, Illinois, a Chicago suburb, is the son of Eleanor (Mueller) and Franz Ferdinand Schlachta, both postal workers. He has two sisters, Marlene, born in 1938, and Heidi, born in 1935. Dennis Franz graduated from Southern Illinois University, whereupon he was promptly drafted into the military. His eleven-month stint in Vietnam, serving in a reconnaissance unit, had a profound impact on his life, leading to depression that lingered for some time after his service.
Following his military service, Dennis Franz joined the Organic Theatre Company in 1972. It was during an audition that he caught the attention of renowned director Robert Altman, who encouraged him to relocate to Los Angeles, where he became a part of Altman's resident company. This marked the beginning of a significant chapter in his career.
In 1982, on April Fool's Day, Dennis Franz met Joanie Zeck, and the two formed a close bond, with Franz playing a crucial role in raising her two daughters, Krista, born in 1976, and Tricia, born in 1974. Thirteen years later, in 1995, they tied the knot in Carmel, California, cementing their lifelong commitment to one another.