A native of Sarasota, Florida, Shields attended Booker High School, where he played football, baseball and ran track. In football, he played wide receiver. During his senior year, Shields had 67 receptions for 1,201 yards and 22 touchdowns. His performance helped his team make it to the Class 3A Championship game. Shields was ranked the 17th best wide receiver coming out of high school by Rivals.com. He received athletic scholarship offers from the University of Miami, Louisville, Louisiana State and Pittsburgh.
Shields also lettered twice in track and field while at Sarasota, where he was one of the state's top sprinters. He recorded a personal-best time of 10.47 seconds in the 100-meter dash, and also ran a leg on the Sarasota 4 × 100 meters relay squad that captured the state title with a time of 41.77 seconds.
Shields ultimately decided to play at Miami, where he started seven games in his freshman year. He recorded 37 receptions, the most by a Miami true freshman since Reggie Wayne in 1997. During his first start against Louisville, he caught three passes for 63 yards including a reception for 49 yards. His first touchdown reception came on a trick play where he caught a 37-yard pass from safety Lovon Ponder versus North Carolina. Shields also scored two touchdowns in a game against Duke. Miami earned a bowl bid to the MPC Computers Bowl against Nevada. In the game, Shields had four catches for 101 yards and caught a 78-yard touchdown pass to win the game. His performance earned him a spot as an honorable mention All-American on the freshman team by Sporting News. He was also named a selection to the 2006 Atlantic Coast Conference Academic Football Team.
During his sophomore season, Shields started five of the ten games that he participated in. On September 20, 2007, in a game against Texas A&M, Shields recorded a career-high 117 yards off six receptions. He finished the season with 27 receptions for 346 yards and three touchdowns.
Shields saw an increase in playing time and played in all 13 games during his junior year either as a receiver or on special teams. He had a season-high three catches for 32 yards against Virginia. He was also voted Miami's Special Teams Player of the Year.
During his final season with the Hurricanes, Shields switched positions from wide receiver to cornerback. During a game against Georgia Tech, he made six tackles. In a match-up between Wake Forest, Shields posted a career-high seven tackles as well as a forced fumble and recovery. He received the Nick Chickillo Most Improved Player award on defense at Miami's awards banquet.
Shields was projected to be a seventh round pick in the 2010 NFL Draft but went undrafted after being arrested for possession of marijuana and signed with the Green Bay Packers for a minimum contract and a $7,500 bonus.
In his rookie year in the NFL, Shields played the nickel back position serving as the third cornerback behind veterans Charles Woodson and Tramon Williams. Shields played in 14 of 16 regular season games, missing two with a calf injury early on. In the regular season, he had 27 tackles (22 solo), with two interceptions and nine defensed passes. During the opening game of the season against the Philadelphia Eagles, Shields and teammate Morgan Burnett became the first rookie defensive back tandem to start for the Packers since the AFL and NFL merged. The first interception of his entire career came against the Dallas Cowboys. He had one more that season against the New York Giants. During the playoffs, Shields added an additional 13 tackles (eight solo) with two interceptions, a forced fumble, and a sack. In the 2011 NFC Championship game, Shields intercepted Chicago Bears quarterback Caleb Hanie in the final minute to send the Packers to Super Bowl XLV. Shields recorded two interceptions, a sack, and a forced fumble in the game, the only rookie in NFL history to do so in a playoff game and only the fifth NFL player to do so since 1982 when the NFL began tracking sacks as an official statistic.
Shields had 2 tackles in the Packers victory over the Steelers in Super Bowl XLV. Shields suffered a shoulder injury late in the second quarter and saw limited play time in the second half.
Shields started 3 games at cornerback in the 2011 regular season and contributed in 15 games overall, intercepting 4 passes and making 45 tackles along with 1 forced fumble.
During the Week 3 Green Bay Packers at the Seattle Seahawks game during the 2012 NFL season, Shields was shoved down during a play by Seahawks wide receiver Golden Tate at the end of regulation play that resulted in a simultaneously catch by Tate and Packers safety M. D. Jennings as ruled by officials and which gave the Seahawks their second victory of the season. The NFL would later release an official statement that acknowledged pass interference should have been called on Tate, but supported the decision to uphold the play.
On March 8, 2014, just days away from becoming a free agent, Shields signed a four-year contract with the Packers for $39 million, including a signing bonus of $12.5 million.
Shields suffered a concussion while making a tackle on Jacksonville Jaguars running back T. J. Yeldon in Week 1 during the 2016 season. This marked his fourth known concussion in his NFL career. After missing the next four games still recovering, he was eventually placed on injured reserve on October 18, 2016.
On February 8, 2017, Shields was released by the Packers after seven seasons with the team.
In October 19, 2016, police found marijuana after searching Shields' home. Shields was officially charged with two counts of marijuana possession on January 16, 2017.