Alexander was born along with his identical twin brother Mackenro in Naples, Florida,[1] to Haitian parents Jean and Marie Alexandre (their children's surname is spelled differently than theirs).

[2] As a senior in 2012, he compiled 42 tackles and three interceptions, helping lead the Indians to an 11–4 mark and a berth in the Florida 5A title game, which they lost, 21–20, to Tallahassee Godby.

For the outdoor team in 2012, he ran a career-best time of 50.39 seconds in the 400-meter dash on his way to a first-place finish at the Collier County Athletic Championships.

He finished his collegiate career at Clemson with 44 total tackles (33 solo), 4 of them for loss of yards, 11 pass deflections and one fumble recovery.

[8] Opposing team's quarterbacks threw a total of 57 passes at receivers covered by Alexander, and just 20 of them were completed, with two touchdowns, none of them over the final nine games of the season.

Alexander had a stellar season for a Tigers team that went 14–1 and made it to the National Championship game in Phoenix, Arizona, earning All-ACC honors after shutting down every receiver he faced.

He injured his hamstring against Oklahoma in the College Football Playoff Semifinal and tried to play through the injury against Alabama, but he aggravated it in the first half and didn't return.

[13] On March 10, 2016, Alexander participated at Clemson's pro day and was one of multiple highly valued prospects to perform for scouts and team representatives.

Four NFL head coaches attended, including Marvin Lewis (Cincinnati Bengals), Chuck Pagano (Indianapolis Colts), Rex Ryan (Buffalo Bills), and Mike Tomlin (Pittsburgh Steelers).

[14] Alexander attended pre-draft visits with the Steelers, New York Jets, New Orleans Saints, Tennessee Titans, Colts, Detroit Lions, San Diego Chargers, and Los Angeles Rams.

Mike Renner of Pro Football Focus (PFF) ranked the Vikings' selection of Alexander as the fifth best in the 2016 NFL draft.

[24] Alexander joined a deep cornerback group that included Xavier Rhodes, Terence Newman, Trae Waynes, and Captain Munnerlyn.

[25] Head coach Mike Zimmer named Alexander the fifth cornerback on the Vikings' depth chart to begin the regular season, behind Rhodes, Newman, Munnerlyn, and Waynes.

[30] Alexander entered training camp slated as the first-team nickelback after Captain Munnerlyn departed for the Panthers during free agency.

[33] On November 12, 2017, Alexander recorded a season-high three solo tackles, two pass deflections, and made his first career interception during a 38–30 victory at the Redskins in Week 10.

Alexander made his first career interception off a pass by Redskins' quarterback Kirk Cousins, that was originally intended for wide receiver Jamison Crowder, and returned it for a ten-yard gain in the second quarter.

On January 14, 2018, Alexander appeared in his first career playoff game and made one tackle and one pass deflection during a 29–24 victory against the Saints in the NFC Divisional Round.

The following week, the Vikings were eliminated from the playoffs after a 38–7 loss at the Philadelphia Eagles who were the eventual Super Bowl LII Champions.

[47] In Week 15 against the Steelers on Monday Night Football, Alexander recorded his first interception as a Bengal off a pass thrown by Ben Roethlisberger during the 27–17 win.

On December 6, 2022, Alexander was signed to the practice squad of the Cowboys, to provide depth after cornerback Anthony Brown was lost for the season with a torn Achilles.