As a freshman, Mannion began playing basketball for Pinnacle High School in Phoenix, Arizona under head coach Charlie Wilde.[2] On February 9, 2017, he featured in a Sports Illustrated article, "A 15-Year-Old (Sorta-Maybe) Basketball Prodigy".[3] Mannion started in all 28 of his games, averaging 20.2 points, 4.6 rebounds, 4.7 assists, and 2.4 steals per game while leading Pinnacle to a 22–6 record.[4] He was named MaxPreps first team All-American and Arizona Republic Class 6A first team All-State.[5] Entering his sophomore season, Mannion was named to the USA Today All-USA Arizona preseason team. He missed his first four games with a broken left hand.[6][7] On February 28, 2018, Mannion scored 21 points in a 76–60 win over Mountain Pointe High School for the Class 6A state championship.[8] He finished the season averaging 23.4 points, 4.7 rebounds, and 5.8 assists per game, leading Pinnacle to a 24–7 record.[9] Mannion received MaxPreps Sophomore All-American third team and Arizona Gatorade Player of the Year distinction.[10]
He took online summer courses to be able to graduate in the following spring.[11] On December 1, Mannion scored 37 points, including the game-winning three-pointer, in an 82–80 victory over Shadow Mountain High School, who had won 73 straight games against Arizona opponents.[12][13] MaxPreps writer Jordan Divens said that Mannion's performance in the game "will go down in Arizona high school basketball lore."[12] Five days later, Mannion recorded 33 points and 12 assists in a nationally televised game against top recruit Jalen Green and San Joaquin Memorial High School at the Hoophall West Invitational.[14] On December 30, he scored 45 points in an 88–78 loss to Mater Dei High School at the Rancho Mirage Holiday Invitational National Division championship.[15] Mannion, on February 2, 2019, tallied 57 points, including the game-winning shot as time expired, to help defeat Chaparral High School.[16] On February 27, he led Pinnacle to a second Class 6A state title, posting 56 points, eight rebounds, and six assists in an 83–64 win over Chaparral High School.[17] By the end of the season, Mannion was averaging 43.4 points, 6.2 rebounds, and 6.2 assists per game.[18] He reclaimed the Arizona Gatorade Player of the Year award and was recognized as National High School Coaches Association (NHSCA) Senior Athlete of the Year.[19][20] Mannion also earned USA Today All-USA second team and Naismith All-American third team honors.[18][21] In the spring, he played in the McDonald's All-American Game and Nike Hoop Summit.[22][23]
Mannion earned preseason first-team All-Pac-12 Conference honors and was named to the preseason watch lists for the Wooden Award and Naismith Trophy.[29] In his second collegiate game for Arizona, on November 6, 2019, he recorded 23 points, nine assists and four rebounds in a 90–69 win over Illinois.[30] On November 28, Mannion posted 16 points and 11 assists and made a game-winning layup with four seconds left in a 93–91 victory over Pepperdine in the first round of the Wooden Legacy.[31] In a second-round win over Penn, he scored a season-high 24 points. Mannion led his team to the Wooden Legacy title and was named tournament most valuable player (MVP) after averaging 16.3 points and 7.3 assists per game. In the following day, he earned Pac-12 freshman of the week accolades.[32] On February 29, 2020, Mannion had 19 points and six assists in a 69–64 loss to UCLA.[33] At the conclusion of the regular season, Mannion was named to the All-Pac-12 second team and the All-Freshman Team.[34] Mannion averaged 14.0 points and 5.3 assists per game as a freshman. Following the season, he declared for the 2020 NBA draft.[35]
On August 10, 2021, Mannion signed a two-year contract with Italian club Virtus Bologna of the Lega Basket Serie A,[38] a team with the ambition of winning the EuroCup and qualify to the EuroLeague. The deal was closed in 24 hours after the end of the free agency.[39] After having ousted Lietkabelis, Ulm and Valencia in the first three rounds of the playoffs, on 11 May 2022, Virtus defeated Frutti Extra Bursaspor by 80–67 at the Segafredo Arena, winning its first EuroCup and qualifying for the EuroLeague after 14 years.[40] However, despite having ended the regular season at the first place and having ousted 3–0 both Pesaro and Tortona in the first two rounds of playoffs, Virtus was defeated 4–2 in the national finals by Olimpia Milano.[41]
On 29 September 2022, after having ousted Milano in the semifinals, Virtus won its third Supercup, defeating 72–69 Banco di Sardegna Sassari and achieving a back-to-back, following the 2021 trophy.[42] However, despite good premises Virtus ended the EuroLeague season at 14th place, thus it did not qualify for the playoffs.[43] Moreover, the team was defeated in the Italian Basketball Cup final by Brescia. In June, after having ousted 3–0 both Brindisi and Tortona, Virtus was defeated 4–3 by Olimpia Milan in the national finals, following a series which was widely regarded among the best in the latest years of Italian basketball.[44][45]
Saski Baskonia (2023)
On June 26, 2023, Mannion signed a two-year contract with Spanish club Saski Baskonia of the Liga ACB and the Euroleague.[46] In twelve league games, Mannion averaged 2.0 points, 0.7 rebounds and 2.0 assists per game, playing 10 minutes per contest. On December 21, Mannion and Baskonia reached an agreement to end the contract.[47]
