He was then signed by the AFL's Denver Broncos and became a starter by the middle of his rookie season. He won All-AFL honors in his second season and played in the AFL All-Star Game, recording 9 interceptions for 144 yards.
In 1967, Brown was traded to the AFL's Oakland Raiders and would spend the remainder of his career there. He served as defensive captain for 10 of his 12 years with the team. He was named to 5 AFL All-Star games and 4 NFL Pro Bowls. He was also named All-AFL 3 times and All-NFL 4 times.
Willie Brown retired after the 1978 season, and finished his Raiders career with 39 interceptions, tied for first all-time on the team. Overall, he finished his 16 Pro Football seasons with 54 interceptions, which he returned for 472 yards and 2 touchdowns. He also recovered 3 fumbles.
Brown served as a defensive backfield coach for the Raiders from 1979-88. He also had stints as the head at Long Beach State, where he earned a master's degree, in 1991 and L.A. Jordan High School in 1994. In 1995 he returned to the Raiders as the Director of Staff Development.