Born in Poway, California, Carson Seymour attended Great Oak High School in Temecula, California, where he was a two-year letter winner and team captain in 2017, his senior year. Transitioning from the infield to pitching that year, the right-hander posted a 1.88 ERA, earning All-Southwestern League and Riverside County All-Star Team honors. A very good student, he went to Dartmouth upon graduating.
In his freshman year at Dartmouth, Seymour made eight appearances as a reliever, allowing 15 earned runs in 7.1 innings, giving up 13 hits, walking 7, and striking out 2. That summer, he played for the Southampton Breakers in the Hamptons Collegiate Baseball League and began considering the possibility of transferring schools.