Nix grew up playing youth baseball as a catcher. He was involved in a home plate collision at the age of 11 that led to chronic pain in his back, and required spinal fusion surgery, which he had at the age of 12.
Nix attended Los Alamitos High School in Los Alamitos, California, and played for their baseball team. He played as a catcher until his freshman year, when he was needed as a pitcher. In his junior year, Nix pitched to a 7–2 win–loss record with a 2.80 earned run average, and was named to the Press-Telegram's all-area first team. He committed to attend the University of California, Los Angeles (UCLA) on a college baseball scholarship to play for the UCLA Bruins baseball team. At the start of his senior year of high school, Nix played for the United States national baseball team in the 2013 18U Baseball World Cup. The United States won the gold medal in the event. By the start of his senior season, his fastball velocity ranged from 92 to 95 miles per hour (148 to 153 km/h).
The Houston Astros selected Nix in the fifth round, with the 136th overall selection, of the 2014 Major League Baseball (MLB) draft. Nix and the Astros agreed to terms on a contract, including a $1.5 million signing bonus, well above the $370,500 recommended at the 136th slot. However, the Astros failed to come to terms with Brady Aiken, the first overall selection of the draft, which reduced their allotted draft pool, leaving them unable to sign Nix without incurring penalties. The Astros reneged on their agreements with Nix and Mac Marshall, and offered Nix a $616,165 signing bonus, which was the most the Astros could offer him without incurring penalties. Nix rejected the offer. He filed a grievance against the Astros, which put his collegiate eligibility in jeopardy. The two sides reached a settlement in December, but did not disclose the terms.
Rather than wait for a ruling on his collegiate eligibility, Nix opted to take a post-graduate year at the IMG Academy in Bradenton, Florida, to play for their baseball team. In this way, he became eligible for the 2015 MLB draft. After watching him throw for scouts, Keith Law, a draft expert for ESPN, wrote that he believes Nix could be a first-round pick. In ten starts for the IMG Academy, Nix improved his fastball velocity to 95 to 97 miles per hour (153 to 156 km/h), improved his changeup, and developed a curveball.
Nix did not consent to being reselected by the Astros in the 2015 draft. The San Diego Padres selected Nix in the third round, with the 86th overall selection, of the draft. In 2016, Nix pitched for the Fort Wayne TinCaps of the Class A Midwest League, where he posted a 3–7 record with a 3.93 ERA.
Nix began the 2017 season with the Lake Elsinore Storm of the Class A-Advanced California League.