Born January 9, 2002, in Houston, Texas, Upton Stout III attended North Shore Senior High School in Houston, where he played cornerback and helped the team win back-to-back Texas 6A-1 state championships in 2018 and 2019. A three-star recruit, Stout committed to play college football for the North Texas Mean Green, becoming a starter midseason during his freshman year in 2020 and posting eleven tackles in his first collegiate start. He appeared in eight games that season, recording thirty-five total tackles before appearing in four games in 2021, totaling six tackles. Stout then redshirted and entered the NCAA transfer portal, eventually committing to the Western Kentucky Hilltoppers.

In his Western Kentucky debut in 2022, Stout announced his arrival with an interception returned for a touchdown, setting the tone for a standout season. He started eleven games, posting forty-four tackles—sixth on the team—and four interceptions, tied for first on the team. Strengths include exceptional speed demonstrated by his 4.44-second forty-yard dash at the NFL Combine, outstanding ball skills and playmaking ability evidenced by multiple pick-sixes, aggressive tackling despite his size, and an impressive thirty-seven-and-a-half-inch vertical jump that aids his coverage ability.

Despite missing five games due to injury in 2023, Stout recorded twenty-nine tackles, eight pass breakups, and another interception returned for a touchdown. He entered the transfer portal after the season but chose to return to Western Kentucky for 2024. In his final college season, Stout earned first-team All-Conference USA honors after totaling fifty-two tackles, seven-and-a-half tackles for loss, an interception, and a sack. He declared for the 2025 NFL Draft and was invited to the Senior Bowl.

The San Francisco 49ers selected Stout in the third round with the 100th overall pick in the 2025 NFL Draft. In his rookie season, he appeared in sixteen games with four starts, recording eighty-two total tackles, one sack, five pass deflections, and one forced fumble. He also appeared in two postseason games, adding seven tackles and one pass deflection. At five feet nine inches and 181 pounds, Stout plays with a physicality that belies his size, demonstrating the same tenacity that helped him win state championships in high school.

Stout has his own clothing line called "Lazy Wayz," showcasing his entrepreneurial interests beyond football.