After being named 2009 Defensive High School Player of the Year by USA Today, he played college football for the Florida State Seminoles and was selected by the St. Louis Rams in the second round of the 2014 NFL draft.

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.

[6] 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.

As a true freshman, Joyner attended Florida State University, where he 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 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.

[11] Head coach Jeff Fisher named Joyner the starting nickelback and the third free safety on the Rams' depth chart to begin the regular season.

[12] He made his professional regular season debut in the Rams' season-opener against the Minnesota Vikings and recorded three solo tackles during their 34–6 loss.

[13] On October 26, 2014, Joyner recorded a season-high ten combined tackles (nine solo) during a 34–7 loss at the Kansas City Chiefs in Week 8.

Joyner missed another two games (Weeks 14–15) as a healthy scratch after defensive coordinator Gregg Williams opted to retain Gaines at nickelback.

Joyner became the de facto nickelback after Gaines sustained a lisfranc injury that was expected to sideline him for the entire season.

On November 9, 2015, Joyner was involved in a controversial play after delivering a late hit on quarterback Teddy Bridgewater in the fourth quarter of the 21–18 overtime loss to the Vikings.

During the play, Bridgewater was performing a baseball slide after running for a five-yard gain and was hit by Joyner and immediately knocked unconscious.

[20] Head coach Jeff Fisher named Joyner the fourth cornerback on the Rams' depth chart to begin the regular season, behind Trumaine Johnson, E. J. Gaines, and Coty Sensabaugh.

[21] On October 16, 2016, Joyner recorded a season-high ten combined tackles (eight solo) and deflected a pass during a 31–28 loss at the Detroit Lions.

[22] In Week 4, he made four solo tackles, deflected a pass, and sacked quarterback Ryan Tannehill in the Rams' 14–10 loss to the Miami Dolphins.

[16] During organized team activities, defensive coordinator Wade Phillips decided to have Joyner work out at free safety.

[26] He started the Los Angeles Rams' season-opener against the Indianapolis Colts and recorded four solo tackles, two pass deflections, and returned an interception by Scott Tolzien for a 29-yard touchdown in the fourth quarter of their 46–9 victory.

[29] On December 3, 2017, he made a season-high three pass deflections, five combined tackles, and returned an interception by Carson Palmer in the Rams' 32–16 victory at the Cardinals in Week 13.

[27] Joyner was inactive for the Rams' Week 17 loss to the San Francisco 49ers, due to the decision by head coach Sean McVay to rest him for the playoffs.