Brown's eight-year professional career was spent exclusively with the Washington Redskins. The team had selected him as an afterthought, in the eighth round of the 1969 NFL-AFL Draft. Though Washington was primarily a passing team, starring All-Pro quarterback Sonny Jurgensen, and in 1967, they had the NFL's first (Charlie Taylor), second (tight end Jerry Smith) and fourth ranked receivers in passes caught, they needed a productive rusher. Brown was an unlikely candidate, having served as a blocking back for Cornelius Davis at Kansas State, where the quarterback, Lynn Dickey, broke all school passing records. Brown had not been widely recruited in high school. His strongest feeler came from Howard University, but upon visiting its campus, he noted the lopsided football scores against the university's teams posted on past schedules in the school's athletic building.
In 1969, newly arrived Redskins coach Vince Lombardi noticed Brown, a talented but underperforming running back. He made the 5'11", 195 pound rookie his starter. But Lombardi noticed Brown was starting slightly late behind the snap of the ball. Tests ordered by Lombardi determined that Brown was hearing-impaired in one ear, and that he was watching for the lineman to move rather than listening to the quarterback's snap count. After getting approval from the league Commissioner's office, Lombardi had Brown's helmet fitted with an ear-piece that relayed quarterback Sonny Jurgensen's snap counts, improving Brown's responsiveness, thus allowing him to hit the hole very quickly. Brown's other rookie obstacle was his training camp propensity to fumble. Lombardi ordered Brown to carry a football everywhere he went at the team's training camp in Carlisle, Pa.
Brown had an impressive rookie season during which Brown was largely the reason Washington posted a record of 7-5-2, their first winning record since 1955. He had rushed for 888 yards, a team record. Lombardi died of cancer during the preseason of Brown's second year. Brown went to four consecutive Pro Bowls during his first four seasons and led the Redskins to their 1973 Super Bowl VII appearance against the "perfect season" Miami Dolphins. Brown was the National Football League's Most Valuable Player in 1972. He was noted for his courageous running style despite his relative small style, courage he attributed to having been raised on the tough streets of Pittsburgh's Hill District, and playing tackle football in those streets. He was also noted for his abilities to break tackles, and gain yardage after contact, which announcers called "second effort".
He finished in the top five of the league for rushes five times, rushing yards three times, yards from scrimmage three times and total touchdowns twice. Brown was the first Redskins running back to gain more than 1,000 yards in a single season. He achieved that feat twice in a career that ran from 1969 to 1976. In an eight-year career, Brown was selected to play in the Pro Bowl in 1969, 1970, 1971, and 1972. He has been voted one of the 70 Greatest Redskins of All Time. He was selected as the Washington, DC Touchdown Club Player of the Year in 1972.
Brown carried the ball 1,530 times in his career gaining 5,875 yards. His best seasons were in 1972 when he gained 1,216 yards and in 1970 when he gained 1,125 yards. He rushed for 100 yards or more 21 times and rushed for 100 yards or more in six games in 1970 and six games in 1972. He also scored four rushing touchdowns in one game against the Eagles on December 16, 1973. . On October 29, 1972, he ran for 190 yards in a game against the New York Giants. Brown wrote an autobiography entitled "I'll Always Get Up".
Brown's career was cut short due to numerous injuries. Brown's jersey number, 43, while not officially retired, has not been issued to any other Redskins player since his retirement.
Brown is currently a Vice President of NAI Michael Commercial Real Estate Services. After retiring from football in 1976, he was employed at E.F. Hutton as a Personal Financial Management Advisor. 
For 12 years, Brown was employed by Xerox Corporation with responsibilities for business and community relations.