Brock Marion was a 5'11" defensive back for the Nevada-Reno Wolfpack. He was selected by the Dallas Cowboys in the seventh round of the 1993 NFL Entry Draft with the 196th overall pick. Marion played 71 games over five seasons for the Cowboys, starting 43 at free safety. He collected eight interceptions, including six in 1995, and 290 tackles.
Miami signed Marion as a free agent in 1998, where he started all 16 games as the Dolphins free safety. He totalled 98 tackles, second only to Zach Thomas on the team. Miami went 10-6, advancing to the second round of the playoffs.
In 1999, Marion started all 16 games for the 9-7 Dolphins, his 86 tackles were again second only to linebacker Zach Thomas. He collected two interceptions and also served as Miami's primary kickoff return specialist, gaining an NFL high 1,524 yards on 62 returns. He also returned seven more kickoffs for 178 yards in two rounds of the playoffs, and intercepted Seattle Seahawks quarterback John Kitna in the wildcard round.
In 2000, Marion earned his first nod for the Pro Bowl, and was also named to the first-team All-AFC team by Pro Football Weekly. His 96 tackles were three behind repeat team leader Thomas, as he again pulled double duty for Miami, intercepting five passes and leading the team with 22 kickoff returns for 513 yards. The 11-5 Dolphins again advanced to the second round of the playoffs.
2001 would see Marion make 79 tackles and five interceptions, leading the NFL with 227 return yards and two touchdowns. In a week 12 41-7 victory over the Indianapolis Colts, he twice picked off Peyton Manning, returning them a total of 78 yards. He also scored his touchdowns on the last two weeks of the regular season, including a 100 yard return in the finale, a 34-7 win over the Buffalo Bills. This push helped the Dolphins to finish out the season 11-5, making the playoffs as a wildcard.
In 2002, Marion was selected to his second Pro Bowl, starting every game for Miami and making five interceptions and 93 tackles. The Dolphins finished 9-7, missing the playoffs.
Marion was selected to his second consecutive Pro Bowl for his efforts in 2003, and third overall. He made 81 tackles and three interceptions as the Dolphins missed the postseason with a 10-6 record.
Detroit would sign Marion prior to the 2004 season, where he would finish out his career after one season. He totalled 533 tackles and 20 interceptions for the Dolphins over six seasons, starting 95 games at free safety.