St-Juste attended various college football camps in the United States in the mid-2010s, and was offered a scholarship to play for the Michigan Wolverines under Jim Harbaugh.[3][6] He played in 12 games for the Wolverines as a freshman in 2017, but redshirted as a sophomore the following year due to a hamstring injury.[7] He graduated from Michigan in two years with a bachelor's degree in sports management and transferred to the University of Minnesota as a graduate student in 2019.[2][3] St-Juste played in 18 games and had 59 tackles in two years for the Minnesota Golden Gophers before finishing his collegiate career.[8] He was invited to the 2021 Senior Bowl, where he recorded a tackle.[9]
St-Juste was selected by the Washington Football Team in the third round (74th overall) of the 2021 NFL draft, in which they took St-Juste with one of the two picks they acquired in a trade that sent Trent Williams to the San Francisco 49ers.[13] He was also selected by the Toronto Argonauts in the sixth round (52nd overall) of the 2021 CFL Draft shortly after.[14] St-Juste signed his four-year rookie contract with Washington on May 13, 2021.[15] He made his NFL debut in the opening game against the Los Angeles Chargers, where he recorded four tackles, but also allowed six catches in a 20–16 loss.[16] He was placed on injured reserve in December 2021 due to lingering issues from a concussion.[17]
St-Juste moved from outside corner to the slot for the 2022 season.[18] He was moved back to outside corner following the benching of William Jackson III in Week 5.[19] With 35 seconds remaining in Thursday Night Football and the Chicago Bears on fourth and goal, St-Juste made a key stop pushing wide receiver Darnell Mooney, who caught the pass, out of the endzone before Mooney's feet landed on the field and sealed the Commanders' 12-7 victory.[20] In Week 9 against the Minnesota Vikings, he deflected a touchdown pass attempt from Kirk Cousins which cornerback Danny Johnson intercepted.[21] The following week's victory over the then undefeated Philadelphia Eagles, St-Juste forced wide receiver Quez Watkins, after catching a 50-yard pass, to fumble the ball allowing safety Darrick Forrest to recover it.[22] On January 6, 2023, he was placed on injured reserve.[23]