Samuelson played for the Mater Dei High School basketball team for three seasons. As a sophomore in 2012–13, she averaged 20. 9 points and 6. 9 rebounds per game and helped Mater Dei to a 30-2 record. The following season, Samuelson averaged 26. 3 points and 9. 4 rebounds. She was named the 2014 California Player of the Year and was a USA Today first-team All-American. As a senior, Samuelson averaged 24. 9 points and 8. 5 rebounds. She helped Mater Dei win their third consecutive Trinity League championship and the 2015 state regional championship; the team finished 31-3. Following the 2014–15 season, Samuelson was named the national player of the year by Gatorade, USA Today, McDonald's, and the Women's Basketball Coaches Association, won the Naismith Trophy, and made the consensus All-American first team. During her high school career, she set Mater Dei single-season records for points average (29. 2), field goal percentage (62. 0), free throw percentage (89. 4), free throws made (178), free throws attempted (211), and three-pointers made (117).