Hart played high school football at Handley High School in Fort Worth. He then played two years of junior college football at Navarro College, after which he walked on to the football team at Arlington State College (now the University of Texas at Arlington), eventually earning a football scholarship. He was not drafted out of college, and was picked up on waivers by the Packers in the 1963 preseason. He spent all of that year on the Packers' taxi squad, but played in every Packers game from 1964 until his retirement after the 1971 season. In his NFL career as a cornerback and safety, Hart had 15 interceptions. Perhaps the most notable of them was his 85 yard interception return for a touchdown against the Minnesota Vikings at Milwaukee County Stadium in 1969—the longest interception return in the NFL that season. As of 2011, his five defensive touchdowns were tied for fourth place all-time for the Packers.
Legendary Packers coach Vince Lombardi signed Hart to play for Green Bay after Hart, having been cut by the St. Louis football Cardinals, had gone to work for the Bell Helicopter Company. After playing for the Packers in an exhibition game in Dallas, the Packers brought him up to Green Bay, where he was pleased to sign a contract: "Lombardi said I was going to be on the taxi squad as a rookie for $500 a week. That was more money than I’d ever seen in my life.” As was the case with many of his players, Lombardi left a lasting impression upon Hart: "I think of Coach Lombardi and his philosophies in one way or another almost every day...He taught us to do your very best at whatever you're doing. He always said, 'When you walk off this field, you want to have those people in the stands say they just saw the very best playing at their very best.' " In a 2013 interview, Hart said of his former coach, "He was a humane person, he really was...He was big and strong and he could get very hard (with people) sometimes, but when a person needed help he was available.” Hart's teammate, guard Jerry Kramer, specifically mentioned Hart in an op-ed article he wrote for the New York Times originally published in 1997: "Max McGee, too, is a wealthy businessman (he founded Chi-Chi's, the chain of Mexican restaurants). So are Paul Hornung, Bart Starr, Doug Hart and a dozen others who didn't leave the game as rich men. All are still driven by Lombardi -- not because he ranted and raved but because he wanted desperately to see us do well." 
After his playing career, Hart was a successful businessman. He was a vice president for snowmobile manufacturer Arctic Cat, and COO of Satellite Industries, a portable toilet manufacturer. He also ran a textile factory and, late in his career, became a licensed fly fishing guide in Florida. He moved back to Minnesota in 2007 to be closer to his children and grandchildren.
^Gerry Fraley, "North Texas pipeline: DFW high schools lay groundwork for elite players to make impact on biggest stage", Dallas Morning News, February 2, 2011. north-texas-pipeline-dallas-fw-high-schools-lay-groundwork-for-elite-players-to-make-impact-on-biggest-stage.ece?nclick_check=1