Johnson, a 6'5" shooting guard from Palma High School in Salinas, California, first played collegiately at Loyola Marymount. As a freshman in the 2007–08 season, Johnson averaged 12.4 points and 4.9 rebounds per game, leading the Lions in both categories. Following a coaching change at LMU, Johnson transferred to UCSB.
After sitting out the 2008–09 season per NCAA transfer rules, Johnson made an instant impact in the Big West Conference for the Gauchos, averaging 18 points and 5.9 rebounds per game. Johnson also led the Gauchos to the 2010 Big West Conference Men's Basketball Tournament title and an NCAA appearance. Following the season, he was named the Big West conference player of the year and an honorable mention All-American by the Associated Press.
As a junior in 2010–11, Johnson averaged 21.1 points and 6.2 rebounds per game and was again named first team All-Big West. He also led the Gauchos to another conference tournament title and NCAA tournament appearance, again earning Big West tournament MVP honors.
Following his junior season, Johnson declared himself eligible for the 2011 NBA Draft, but opted to return to UCSB for his senior season.
Prior to the start of his senior season, Johnson was named to the preseason watch lists for the Wooden Award Naismith Award, and the Lowe's Senior CLASS Award.
Following his junior season at UCSB, Johnson was chosen to represent the United States as a member of Team USA at the 2011 World University Games in Shenzhen, China. He played in all 8 matches of the tournament, averaging 7.3 points and 3.9 rebounds per game as the team finished 7-1.
Johnson was selected as the 36th overall pick in the 2012 NBA Draft by the Sacramento Kings. He was immediately traded to the Indiana Pacers. During his rookie season, Johnson spent time with the Fort Wayne Mad Ants of the NBA D-League.
Johnson's mother was murdered when he was a year old, and the grandmother who took him in watched her house burn down six years later. Four more of Johnson's family members perished in that blaze, and his grandmother died when he was 11. Johnson's two older brothers raised him afterwards and ensured that Johnson graduated from college.