In the first round of the 2020 NBA Draft, Killian Hayes was selected by Detroit Pistons with No. 7 pick.

The French national had an unconventional route going to the big leagues as he spent most of his young career playing overseas and skipping to see action in the collegiate ranks. Hayes was born in the United States but grew up in France due to his father's professional career. At a young age, he began playing for Espoirs Cholet in LNB Espoirs, the French under-21 league, in the 2016-17 season where he was five to seven years younger than his counterparts.

Hayes eventually debuted for Cholet's senior team at age 16 where he struggled after averaging 2.2 points and 1.2 assists in 9.1 minutes per game through nine appearances. He saw a bigger role in the team in the following season where he tallied norms of 7.1 points and 3.1 assists per game through 34 appearances including a season-high 17 points.

In the 2019-20 season, Hayes signed a three-year contract with ratiopharm Ulm of the Basketball Bundesliga (BBL) which served as the top league in Germany and the EuroCup. He concluded the season posting averages of 11.6 points, 5.4 assists, 2.8 rebounds and 1.5 steals per game through 33 combined games in the EuroCup, BBL and BBL-Pokal.

Killian Hayes was also a mainstay on the French national team where he was named as the Most Valuable Player of the 2017 FIBA U16 European Championship in Podgorica, Montenegro. He continued to showcase his wares at the 2018 FIBA Under-17 Basketball World Cup in Argentina where he won a silver medal and was named to the All-Star Five after averaging 16.1 points, 3.6 rebounds, 3.3 assists and 2.7 steals per game.