Holahan was raised in Connecticut. After graduating from Yale in 1965, Holahan served as an officer (Lieutenant, J.G.) in the United States Navy on an amphibious assault ship in Vietnam from 1967 to 1969. He was awarded the Vietnam Service Medal with a silver star for five campaigns, a Meritorious Unit Commendation, the Republic of Vietnam Meritorious Unit Citation, and the Republic of Vietnam Campaign Medal.
He has two children, Nicholas Drew Holahan and Belle Holahan Casares, from his first marriage to Wylie O'Hara (1966–1972), and seven grandchildren. Holahan was married to Actress Loretta Swit from 1983 to 1995. Holahan is active in the area of drug and alcohol recovery and serves on the board of McIntyre House, a non-profit recovery house for men recovering from alcoholism and drug addiction located in Los Angeles. Holahan also serves on the board of Worth Our Weight, a non-profit organization in Santa Rosa, California which trains and educates teenagers and young adults who have survived foster care programs and teaches them how to work in the fine dining industry.
Holahan graduated from Hastings College of Law in 1973. At Hastings he was the recipient of the Hale Moot Court Prize for Best Brief, was an Editor of the Hastings Law News, 1972–1973, and a founder of the Hastings Child Care Center. After Hastings he worked for a litigation firm in the financial district in San Francisco for two years.
From 1976 to 1992, he took a hiatus from his law practice to pursue a successful career as an actor in films and television. Holahan is best known for appearing with Al Pacino in the crime film Scarface as Jerry the Banker. Notable other roles included Collision Course, Kuffs, Haywire (mini-series) and Aspen Extreme, as well as television appearances in M*A*S*H, Hill Street Blues, Lou Grant and The Rockford Files. He also appeared in the movie The Vegas Strip War in 1984 playing Jimmy Weldstrom. He played a Lawyer in his 1981 appearance on The White Shadow. His character interviewed James Hayward (Thomas Carter) for a summer job.
In 1992 Holahan went back to the legal career he had put on hold in 1976. He started the Law Office of Dennis Holahan, which was based in Los Angeles and specialized in entertainment law, winning several high profile cases.
