In college football, he played running back for the University of Georgia, where he was an All-American and won the 1982 Heisman Trophy. Herschel Walker's freshman season in 1980 is widely regarded as one of the best seasons ever by a first year player. His college career is also widely considered to have been one of the best in American college football history. In its list of the 100 greatest college football players ever, CollegeFootballNews.com placed Walker at

In the formation of USFL, he saw an opportunity to do something then forbidden by NFL rules, namely turn professional after the end of his junior season rather than waiting for his collegiate class to graduate (four years after the high school graduation of his peer group). He also sought to choose where he would play professionally, as he felt he could make considerable money from product endorsements, being quoted on one occasion as saying: "I don't know if I would want to play in the NFL unless it was for the two New York teams or the Dallas Cowboys."

With endorsement considerations in mind, Walker signed with the New Jersey Generals in 1983, owned by Oklahoma oilman J Walter Duncan. (Duncan would sell the team to real estate mogul Donald Trump after the 1983 season) Although this move was challenged in court, Walker and the USFL prevailed and Walker began play with the Generals, winning the USFL rushing title in both 1983 and 1985 and in the latter year also gaining over 4,000 yards. He holds the Pro Football Record for Single-Season Rushing Yards with 2500 yards in 1985.

The NFL's Cowboys suspected that the USFL was not going to last. They acquired Walker's NFL rights by selecting him in the fifth round of the 1985 draft. When the USFL in fact succumbed after its essentially unsuccessful antitrust suit against the NFL in 1986, Walker went to play for the Cowboys, eventually establishing himself as a premier NFL running back with two consecutive Pro-Bowl seasons (1987,1988). In 1989, at the height of his NFL career, the Cowboys traded his rights to the Minnesota Vikings in exchange for a total of five players. This was judged to be one of the seminal events in the return of the Cowboys to the top echelon of the NFL and the trade would ultimately overshadow Walker's NFL career achievments.

Walker did quite well by most standards for several seasons with the Vikings before his rights were later acquired by the Philadelphia Eagles, and, subsequently, the New York Giants. Eventually and ironically, he was acquired by the Cowboys again. In this second tour with the Cowboys, he was used as a flanker and other offensive positions, not exclusively at running back. In addition to running and catching passes, Walker was also often used to return kickoffs throughout his career.

If Walker's USFL and NFL numbers are combined, he ranks as one of the most productive professional football runners in history. Many are reluctant to do this, however, because of the disagreement as to whether the level of play of the USFL was in any way comparable to that of the NFL.

Even without taking his USFL numbers into account, his NFL stats are exceptionally high. In his 12 NFL seasons, Walker gained 8, 225 rushing yards, 4,859 receiving yards, and 5,084 kickoff return yards. This gave him an impressive 18,168 total combined net yards, ranking him high among the NFL's top 20 all time leaders in that category at the time of his retirement. He also scored 82 touchdowns(61 rushing and 21 receiving)

Even so, most football experts rank Walker's professional career as something of a disappointment. This must be due to the high, perhaps unrealistic expectations that were placed upon him due to his college career and the scrutiny which was entailed by so much being traded for him by the Vikings. Additionally, he was never on a championship NFL team. Some observers, however, claim that Walker was at least in part a victim of the package of rules changes the NFL had adopted in the spring of 1978, which were widely assumed to discourage offenses from running (rather than passing) most of the time; in addition, the NFL teams on which Walker played seldom used the I-formation, out of which Walker ran so successfully throughout his career.

Walker competed in the 1992 Winter Olympics in bobsled, finishing seventh. He also won back-to-back American Superstars competitions in 1987 and 1988. He is a sixth-level black belt in tae kwon do. He is a born-again Christian who frequently talked about his faith during his USFL interviews.

On the Fox Sports Net show Sports List, Walker was named the best college football running back of all time, beating out the likes of Jim Brown and Barry Sanders.