On June 21, 2018, Wagner was selected with the 25th overall pick in the 2018 NBA draft by the Los Angeles Lakers. On July 1, he signed his rookie scale contract with the Lakers. On July 10, Wagner injured both his left knee and left ankle in the Las Vegas Summer League. He was sidelined for all of training camp and the preseason by the knee injury. Wagner was assigned to the South Bay Lakers on rehab assignment on October 26. Wagner made his G-League debut for the South Bay Lakers on November 3, scoring 17 points in 29 minutes in a 108–106 loss to the Stockton Kings. He made his NBA debut on November 17 against the Orlando Magic. On December 2, in a 120–96 victory against the Phoenix Suns, Wagner scored his first NBA points (a pair of free throws) and finished the game with 10 points. On March 9, 2019, Wagner scored a season-high 22 points with six rebounds, three assists and a block in his first career start in a 107–120 loss to the Boston Celtics.