Born in Pawtucket, Rhode Island of German descent, Hartman attended Mount Hermon School (now Northfield Mount Hermon) and was geared toward professional baseball in high school. However, he turned down a baseball scholarship to attend Duke University where he majored in economics and became a member of the Sigma Chi fraternity. After college, he served three years active duty as an officer in the U.S. Air Force, Strategic Air Command. He was a Supply Officer at Dow AFB in Bangor, ME and acted in local musicals there, including Oklahoma, in the role of Curly.

Hartman appeared in two Broadway shows: the original Hello, Dolly! in 1964, and The Yearling (1965). After working in films such as The Ballad of Josie (1967) and Did You Hear the One About the Traveling Saleslady? (1968), he refocused on television, and won serious attention as a dedicated doctor on The New Doctors, winning a nomination for a Golden Globe award.

He also appeared as the character David Sutton in more than two dozen episodes of the television series The Virginian. Hartman had guest-starring roles on a number of other popular TV series such as Marcus Welby, M.D., The Name of the Game and Ironside.

He starred in the Disney movie The Island at the Top of the World (1974) as an archaeology professor. A year earlier, Hartman did a remake of the holiday classic Miracle on 34th Street along with Jane Alexander and Sebastian Cabot.

On the 1974-75 NBC series Lucas Tanner, Hartman played a retired baseball player turned unconventional high school teacher. The cancellation of this series marked the end of his acting career.

In November 1975, Hartman became the first co-host of ABC's new show Good Morning America (1975—1987). During his 11 years as host, GMA became the highest rated morning news program. He conducted more than 12,000 interviews.

Hartman has been an anchor and host of a series of documentaries on the Discovery Channel and PBS member station WNET in New York City. Produced by James Nicoloro, the PBS documentaries are a series of "Walk Through" documentaries about various communities around New York City, which include A Walk Down 42nd Street (August 1998), A Walk Up Broadway (March 1999), A Walk Through Harlem (December 1999), A Walk Around Brooklyn with David Hartman and Historian Barry Lewis (2000), A Walk Through Greenwich Village (2001), A Walk Through Central Park (2001), A Walk Through Newark (2002), A Walk Through Hoboken (2003), A Walk Through Queens (2004), A Walk Through the Bronx (2005), and A Walk Around Staten Island (2007).

In North Carolina, Hartman is also heard on North Carolina Public Radio and WCPE-FM as host of the North Carolina Symphony radio broadcasts.

Hartman was married to Maureen Downey from 1974 until her death on September 17, 1997. In 2001, he married Mary Clark Putman, a widowed homemaker.