According to ESPN, he ended his junior year of high school as the number one point guard in the country and prior to his senior year he was rated number one by Scout.com. He was selected as the 2014 Illinois Boys Basketball Gatorade Player of the Year. He was selected to participate in the 2015 McDonald's All-American Boys Game as well as the 2015 Jordan Brand Classic and to represent the Team USA in the Nike Hoop Summit. As a senior, he repeated as the 2015 Illinois Gatorade Player of the Year and earned a third Associated Press All-state recognition (2nd first team). That year, he led Stevenson to the Illinois High School Association (IHSA) Class 4A championship. Following the season, he won Illinois Mr. Basketball.
Brunson holds the IHSA playoff single-game and the IHSA Class 4A championship game scoring records and set the USA Basketball single-game assist record for the FIBA Americas Under-18 Championships. He earned the MVP of the 2015 FIBA Under-19 World Championship for the gold medal winning Team USA. He is the son of nine-year National Basketball Association (NBA) veteran Rick Brunson.
He began his college career as the 2015–16 Big East Conference Preseason Freshman of the Year and was a Big East All-Freshman Team selection following the season. As a starter, he helped the 2015–16 Wildcats win the championship game of the 2016 NCAA Men's Division I Basketball Tournament. As a sophomore, he was a unanimous 2016–17 All-Big East first team selection for the 2016–17 Wildcats.
Born in New Brunswick, New Jersey, Brunson was raised in southern New Jersey until sixth grade. He is the son of Rick and Sandra Brunson. He has a sister Erica (born c. 2001). His parents met at Temple University where Rick played for the Owls men's basketball team and Sandra played volleyball. Rick went on to spend nine seasons in the NBA. The family first settled in Cherry Hill, New Jersey but moved seven times before settling in Lincolnshire, Illinois in 2010 where Jalen played his high school career for Stevenson High School.
As a freshman, Brunson was an All-Lake County honorable mention selection in 2012, as Stevenson finished the season with a 17–11 record. During his sophomore season, Stevenson started the season 10–4 before going on a 19-game winning streak. That year, Brunson led Stevenson to the March 16, 2013 IHSA Class 4A championship game against Jabari Parker and Kendrick Nunn's three-time defending state champion Simeon Career Academy, where he got the Simeon backcourt in foul trouble in the first half before being held to one point in the second half of a 58–40 loss. Stevenson finished the season 29–5. Following the season, the Associated Press named him to the Class 4A All-state second team as the only sophomore on the first or second team. Brunson averaged 21.5 points that season.
On February 21, 2014, junior Brunson scored 57 points in a double overtime victory over Lake Forest High School. The performance gave him both the school single-game and career scoring records. On March 21, Brunson set the IHSA playoff single game scoring record against the Jahlil Okafor-led Whitney Young High School by scoring 56 points in a 75–68 state playoff semifinal loss. In the game, still images of a movement by Brunson appeared to be an obscene gesture, but video of the gesture were deemed to be inconclusive and an initial suspension for the subsequent consolation game was overturned. Brunson averaged 26.1 points, 5.4 rebounds, 4.7 assists and 2.9 steals for a Stevenson team that finished the season with a 32–2 record. Two members of his high school team were National Football League athletes' offspring: Matt Morrissey and Cameron Green. Brunson was named Gatorade Player of the Year for the state of Illinois as a junior. Following the season, Associated Press named him as the only non-senior on the Class 4A All-state first team that also included Okafor, Cliff Alexander, Tyler Ulis and Sean O'Mara. He finished fourth in the 2014 Illinois Mr. Basketball voting. In April 2014, Illinois, Purdue, Kansas and Villanova had in-home visits. On May 3, 2014, he announced the eight schools that he was considering playing college basketball for: UConn, Michigan State, Illinois, Kansas, Purdue, Villanova, Michigan and Temple. That summer he was the number one rated point guard according to ESPN, although the class had a notable shortage of elite pure point guards. On June 25—the day after Jalen won a gold medal at the FIBA Americas Under-18 Championship—Rick Brunson received an offer as an on-bench assistant coach from Temple. The Temple offer fell through when Rick Brunson was arrested on various charges on July 25, and Temple fell out of the running for Jalen's services. Brunson participated in the July 9–11 LeBron James Skills Academy. On August 5, he announced official visits to Illinois on August 29–31, Villanova on September 4–6 and then Temple from September 11–13 and on August 9, he announced official visits to Michigan State from September 19–21 and Purdue on September 26–28.
On September 8, Brunson announced that he would be curtailing his recruitment and making his decision between Illinois and Villanova on September 10. On that date, Brunson committed to Villanova. At the time of his decision, he was ranked as the number one point guard in the national class of 2015 by Scout.com. Stevenson was a preseason top-10 team in the MaxPreps national high school rankings. A lot was expected of Brunson's team after losing to teams led by Parker and Okafor in the IHSA tournament final four in 2013 and 2014 even though no team from Lake County has ever won a state basketball championship. The team rattled off 11 straight victories to start the season before losing 88–81 to Chaminade Prep and its star Jayson Tatum in the Cancer Research Classic in Wheeling, WV, despite 48 points from Brunson. On January 17, Brunson and Stevenson faced Derryck Thornton, Jr. and Findlay Prep at the Hoophall Classic. At the time, Brunson was the number 3 ranked point guard in the class of 2015 and Thornton was the number 2 ranked point guard in the class of 2016 for the number 1 team in the country according to USA Today. Findlay, who was led by Arizona signee Allonzo Trier's 27 points, held Brunson to 26 in the second loss of the season for Stevenson, who had been 15–1. On January 28, he was named to the 2015 McDonald's All-American Boys Game roster, becoming the second player from Lake County to earn such recognition. On February 2, he was named to the 11-man Team USA for the Nike Hoop Summit. On February 21, Stevenson lost to Simeon as Brunson posted 25 points on 9–24 shooting, ending a 22-game in-state streak in a game that featured the top two teams in the state. The Simeon frontline featured three Big Ten Conference signees: D. J. Williams (Illinois), Ed Morrow (Nebraska) and Isaiah Moss (Iowa). On March 5, he was named to the Jordan Brand Classic roster.
On March 17, Brunson helped Stevenson qualify for a third consecutive appearance in the IHSA final four with a victory over Riverside Brookfield Township High School. In the game, Brunson became the Lake County career scoring record holder. On March 19, Brunson received the highest vote total for the Illinois Class 4A Associated Press All-state team. On March 20, he repeated as Illinois Gatorade Player of the Year. Brunson led Stevenson to the IHSA final four for a third consecutive time, but he led the team to the first state championship by a Lake county school with an IHSA Class 4A title-game record 30 points in a 57-40 victory over Normal Community High School. Brunson was 9-for-15 from the field and 9-for-9 from the free throw line. 2015 was Lake county's ninth consecutive year with a team reaching the IHSA final four; Stevenson became the first school in the large school division and the third school overall to win IHSA football and basketball state championships in the same year. The win got him out of the shadow of IHSA final four losses to Parker- and Okafor-led teams. Following the season, he won Illinois Mr. Basketball with 552 points and 99 of the 132 first place votes ahead of fellow Jordan Brand All-American selection Charles Matthews who had 157 points. Brunson finished his senior season with averages of 23.3 points, 5.2 assists and 4.7 rebounds per game and with shooting percentages of 38% on three point shots and 83% on free throws. At the March 30 POWERADE Jam Fest associated with the McDonald's All-American game, Brunson won the skills competition over finalists Carlton Bragg and Isaiah Briscoe and was a finalist for the three-point shooting contest. Brunson was a first-team Parade All-American, as well as a third-team USA Today All-USA high school basketball team selection. In the April 11 Nike Hoops Summit, Brunson had 12 points and 7 assists in a 103–101 loss. In the April 17 Jordan Brand Classic, Brunson had a game-high 3 steals. He finished his high school career ranked #16 in the ESPN 100 and as the #2 point guard (behind Isaiah Briscoe).
Brunson, like his father, is a left-handed basketball player. In what is regarded as a weak point guard class, Brunson is the only true point guard that was ranked in the top 25 players at the conclusion of the class of 2015's junior season. In addition to high ratings by the recruiting services, Brunson's peers voted him to be the best passer in high school basketball prior to his senior season. At the time of his September 2014 commitment to Villanova, he was described by ESPN writer Reggie Rankin as "a left-handed, pass-first lead guard that excels at running his team and can also score as needed". ESPN's Paul Biancardi also used the "pass-first point guard" description.
Brunson was a selection to the 20-man Bob Cousy Award preseason watchlist. He was also selected as the 2015–16 Big East Conference Preseason Freshman of the Year by the conference coaches although Henry Ellenson was the only freshman selected to the 2015-16 Preseason All-Big East First or Second Team. Brunson's 2015–16 Wildcats were the unanimous coaches preseason selection to win the conference. In preseason top 100 player rankings Brunson was ranked 46 by ESPN and 41 by NBC Sports. During the preseason, Mike Rutherford of SB Nation selected Brunson as one of its 10 most important college basketball players for 2015-16. He made the initial 50-man John R. Wooden Award watch list on November 17. On December 2, Brunson earned recognition on the 18-man Wayman Tisdale Award watchlist.
Brunson opened the season in the starting lineup with 12 points and 4 assists against the Fairleigh Dickinson Knights on November 13. On November 26 in the semifinals of the NIT Season Tip-Off against Stanford, Brunson posted 18 points. On December 28, he posted a career-high 22 points against Penn. The 22-point effort was part of a 3–0 week for Villanova in which Brunson averaged 15.3 points and earned Big East Freshman of the Week. On February 8, the 2015–16 Wildcats became the first Villanova Wildcats men's basketball team to reach number one in the AP Poll by climbing to the top of the 2015–16 NCAA Division I men's basketball rankings. Following the 2015–16 Big East season, he was a unanimous Big East All-Freshman Team selection. In the 2016 NCAA Men's Division I Basketball Tournament regional final against Kansas Brunson made the final two free throws that gave the Wildcats a two-possession lead with 3.5 seconds left. The 2015–16 Wildcats won the championship game of the tournament by defeating the North Carolina Tar Heels 77–74, with Brunson as a starter.
Brunson was a selection to the 20-man Bob Cousy Award preseason watchlist again as a sophomore. He was a preseason All-Big East honorable mention selection. On December 5, the 2016–17 Wildcats ascended into the top position in the 2016–17 NCAA Division I men's basketball rankings. The following night, Brunson posted a then career-high 26 points in a Philadelphia Big 5 win over La Salle. After helping guide Villanova to a victory in its 2016–17 Big East season opener against DePaul on December 28, Brunson scored a career high 27 points in a 80–70 December 31 win over No. 10 Creighton to propel No. 1 Villanova to a 14–0 record and 2–0 in conference play. These two performances earned Brunson Big East player of the week recognition on January 2. Brunson was named to the February 9 Naismith Award Top 30 watch list. On February 18 against Seton Hall, Brunson posted a career-high 10 assists and his first career double double. Following the regular season, he was one of four unanimous selections to the 2016–17 All-Big East first team. After averaging 14.7 points and 4.1 assists as a sophomore, Brunson decided to return to Villanova for his junior season. After the season ended and the 2017 NBA draft class was finalized, Brunson was projected to be a preseason All-American by NBC Sports.
On May 5, 2014, USA Basketball announced the 21 athletes (including Brunson) invited to try out from June 10 to 19 for the 12-member USA national team for the June 20–24, 2014 FIBA Americas Under-18 Championship. Eventually, 24 players tried out for the team and the roster was cut to 15 on June 12. Brunson made the final 12-man roster that was announced on June 15. In the opening game, Brunson surpassed Stephon Marbury's 12 assists against Brazil in the 1994 FIBA Americas U18 Championship by recording 13 against Uruguay to set a new USA Basketball U18 single-game assist record. The United States claimed a gold medal in the tournament. On August 16, 2014, Brunson was named to the Nike Global Challenge USA All-Tournament team along with Stephen Zimmerman, D. J. Hogg, Malik Monk, Jaylen Brown and Edrice Adebayo. Brunson led the midwest team to a third-place finish in the eight-team tournament.
On June 18, 2015 Brunson was announced as a member of the 12-man 2015 USA Basketball Men’s U19 World Championship Team for the 2015 FIBA Under-19 World Championship. Brunson earned MVP of the tournament after leading the team with 5.6 assists and 2.1 steals for the tournament. He posted a game-high 30 points in the semifinals against Greece, and he tallied a team-high 14 points including 6 in overtime as well as 7 assists, 5 rebounds and a steal in the gold medal game against Croatia. Brunson tied teammate Harry Giles with a 14.0 average for the tournament. He dominated in the final two games. Based on this performance, he was recognized as the USA Basketball Male Athlete of the Year on December 21, 2015.