A native of Miami, Florida, Joyner originally attended Southwest Miami High School, where he played three seasons on the varsity football team at linebacker and defensive back as well as wide receiver and kick returner. As a sophomore, he had more than 100 tackles and also accounted for more than 1,000 yards on offense. In his junior year, he recorded 112 tackles, four sacks and an interception. He then decided to transfer to St. Thomas Aquinas High School in Fort Lauderdale, Florida, a private school and football powerhouse coached by George Smith. Featuring talented running backs Giovani Bernard and James White, St. Thomas Aquinas went undefeated through the season, extending a win-streak that began in 2007 to 37 wins, until losing 28–20 to Manatee High School in a Class 5A state semifinal at Hawkins Stadium in Bradenton, Florida. For the season, Joyner contributed as a shut-down cornerback, recording only 14 tackles (12 solo) as opponents stayed away from his side of the field. He also starred on special teams, as he averaged 39.6 yards per return and managed to return three kickoffs for scores. On offense, he recorded 1,090 all-purpose yards on just 79 touches, scoring four rushing and four receiving touchdowns. His impact on the defensive side, however, earned him the National Defensive Player of the Year award by USA Today.
He was also on the school's track & field team. He placed 3rd in the 100 meters at the 2010 BCAA Championships, with a career-best time of 10.83 seconds. He was also a long jumper and high jumper.
Despite lacking prototypical size, Joyner was regarded as a five-star recruit by Rivals.com, and ranked as the No. 1 cornerback prospect in his class, ahead of fellow five-star DeMarcus Milliner. With offers from virtually every school in the country, Joyner took official visits to only Ohio State, Notre Dame, and Florida State, before committing to the Seminoles in early December 2009. He was the highest-rated Florida State recruit of the season, along with Christian Jones. Joining Derrick Brooks (1990), David Warren (1996), and Antonio Cromartie (2002), Joyner became the fourth ever USA Today National Defensive Player of the Year to sign with Florida State. He capped off his prep career by playing in the Under Armour All-America Game.
As a true freshman, Joyner played in all 14 games and finished with 23 tackles, two pass break-ups, and one interception, mostly on special teams but also as a back-up safety. In the 47–17 loss to Oklahoma, he had five returns for 74 yards. In the ACC Championship Game against Virginia Tech, Joyner had four returns for 112 yards. During spring practice of his sophomore year, Joyner was moved into the starting line-up at safety, a position at which he started all 13 games of the season. He recorded 54 tackles adding two tackles for loss, one sack, and three pass break-ups on the year, which earned him All-ACC second team honors. In the Champs Sports Bowl, Joyner helped the Seminoles to an 18–14 win over Notre Dame tallying seven tackles and snagging his team-high fourth interception. In his junior year, Joyner again started all 14 games at safety. In his final season at FSU, Joyner had 69 tackles with seven tackles for a loss, two interceptions, four passes broken up, three forced fumbles and 5.5 sacks. He was a finalist for the Jim Thorpe Award as a senior.
Joyner was drafted by the St. Louis Rams in the second round (41st overall) of the 2014 NFL Draft. On the November 9, 2015, Joyner was involved in a late hit on quarterback Teddy Bridgewater in the 21-18 overtime loss to the Minnesota Vikings, Joyner would subsequently be fined $23,152 by the NFL.
In April 2017, it was announced that Joyner would be seeing time at free safety, having primarily playing as a slot corner.
On September 10, 2017, against the Indianapolis Colts in the season opener, Joyner picked off quarterback Scott Tolzien and returned it for a touchdown in the fourth quarter.