He did not play organized basketball until eighth grade, but by late in his freshman season he became a notable contributor to his high school's varsity team.

Following a Martin Luther King Day performance during his senior year against the number one team in the country, many experts considered him to be the best player in the national class of 2014.

Alexander led Curie to the 2014 Chicago Public High School League city championship in a quadruple-overtime game against Okafor that was the lead story on SportsCenter.

As a freshman with Kansas, he did not play in the latter part of the season after the National Collegiate Athletic Association (NCAA) launched an investigation into his eligibility.

[3] On March 9, 2011, Curie was eliminated from the IHSA playoffs in a Class 4A sectional semifinal by Lyons Township High School as freshman Alexander posted 9 points, 13 rebounds, and 2 blocks in a 66–64 loss.

[21] Following the season, a statewide panel of sportswriters and broadcaster voted Alexander as an Associated Press boys basketball Class 4A all-state honorable mention honoree.

[24] By mid-July 2012, as ESPN's number 10 ranked prospect, he received an offer from Kentucky and was being recruited by DePaul, Indiana, Ohio State and Wisconsin.

[28] On November 30, at a Draffenville, Kentucky tournament game against Oak Hill Academy, Alexander scored 26 of his team's 39 points on 12–14 field goal shooting in a 72–39 defeat.

[32] Despite 11 points, 9 rebounds and 8 blocked shots from Alexander, Curie was eliminated from the CPL playoffs in a February 11 quarterfinals 60–58 loss to the Billy Garrett, Jr.-led Morgan Park High School[33] that went on to become 2013 IHSA Class 3A state champion.

[45] Alexander was recognized as an Associated Press All-State Class 4A first team selection along with Parker, Okafor, Malcolm Hill and Sterling Brown.

[49] He also stood out at the July Under Armour Summa Jam, where his Mac Irvin Fire opposed teams led by Myles Turner and Karl-Anthony Towns.

[53] In September, Alexander believed that he would make his final selection with a December verbal commitment and an April signing of his National Letter of Intent.

[59] On October 31, Alexander officially eliminated Michigan State from his candidate list, leaving Kansas, Illinois, DePaul and Memphis in contention.

[68] Prior to his senior season, USA Today named him to its 10-man preseason All-USA team along with Stanley Johnson, Jones, Trey Lyles, Emmanuel Mudiay, Malik Newman, Okafor, Oubre, D'Angelo Russell, and Turner.

[69] Alexander was joined on the Chicago Tribune's preseason Illinois Mr. Basketball top 5 by Okafor, Brunson, Ulis and Keita Bates-Diop.

Alexander almost had to serve a one-game suspension from the game due to having been assessed two technical fouls on December 1 against St. Rita High School.

[76] Some of the stats that Alexander posted were so impressive as to cause disbelief to some, including Chicago Tribune high school basketball writer Mike Helfgot and several of his associates.

[77] In early January, Alexander continued to be a leading contender for national player of the year, holding on to second place in the Mr. Basketball USA race.

[78] On January 10, Alexander led Curie past Villa Angela-St. Joseph High School, which featured Rivals.com Class of 2015 number 10 ranked Carlton Bragg.

[79][80] In the January 20 Hoophall Classic, Alexander led Curie to a victory over D'Angelo Russell, Ben Simmons and Montverde Academy, the number one rated team in the country.

Alexander had 16 points, 12 rebounds, and 6 blocks earning a February 19 CPL semifinals rematch against Hyde Park Academy High School who they have already beaten twice this season.

[94][95] The matchup was the most anticipated city championship game since at least 1998 when fellow McDonald's All-Americans Quentin Richardson of Young and Corey Maggette of Fenwick High School were both top 15 players.

Young and Curie were both in the Marist Class 4A sectional in the 2014 IHSA playoffs and could have met again in the round of sixteen on March 14 if they had both won three regional contests.

[111] In the April 2, 2014, McDonald's All-American Game Alexander posted 9 points and a game-high (tied with Kevon Looney) 11 rebounds for the losing east team.

[152][153] Alexander made his second start of the season on February 10 against Texas Tech, replacing Jamari Traylor in the lineup and posting 10 points, 4 blocks and 5 rebounds.

[166] Alexander missed the entire preseason with the Trail Blazers due to his knee but made the roster and entered the season behind Meyers Leonard, Mason Plumlee, Ed Davis, Noah Vonleh, and Al-Farouq Aminu on the depth chart.

The 10-day contract had been guaranteed and earned Alexander a year of service (toward payscale minimums and pension eligibility) in the NBA making him a 2-year veteran.

[198] On December 19, 2024, Alexander joined the Anyang Jung Kwan Jang Red Boosters of the Korean Basketball League to replace Michael Young.

[201] In the September 26–29, 2013 FIBA 3x3 U18 World Championship in Jakarta, Indonesia, Team USA finished 9th with a 7–1 record, losing in the opening round of 16 in the playoffs.

Alexander picked up basketball late due to the lack of available safe courts to play on in his Chicago West Side neighborhood known as the Brian Piccolo community.