He has previously played for the Detroit Pistons, the team that selected Johnson in the second round of the 2005 NBA draft, as well as the Toronto Raptors, Boston Celtics and Philadelphia 76ers.

He made up for lost time as a senior in 2004–05 as he was named the 2005 California Mr. Basketball after leading Westchester to a state title.

[2] Despite committing to play for the University of Louisville, Johnson decided to skip college and declared for NBA draft.

[10] He was later traded to the Toronto Raptors on August 18, 2009, along with Sonny Weems, in exchange for Carlos Delfino and Roko Ukić.

[12][13] Johnson had a career-best season in 2012–13 as he averaged career-highs in minutes played (28.7 mpg), points (10.0 ppg), rebounds (7.5 rpg), assists (1.5 apg) and steals (1.0 spg).

[16][17] He made his debut for the Celtics in the team's season opener against the Philadelphia 76ers on October 28, recording 15 points and 7 rebounds off the bench in a 112–95 win.

[24] On February 22, 2019, he volunteered to play for the Delaware Blue Coats of the NBA G League because he had not appeared in a game for the 76ers since January 26.

[25][26] On April 13, 2019, during a nationally televised playoff game between Philadelphia and the Brooklyn Nets, Johnson was caught by ESPN cameras sitting on the bench during the game taking a cell phone out of his pocket to show teammate Joel Embiid a text message, while the Sixers were trailing by ten points.

[27] At the post game press conference, Embiid stated that Johnson's daughter was very ill and he just looked at the message.

After the game, Johnson issued an apology through a statement and was fined an undisclosed amount by the 76ers for violating the NBA's rule against cell phones on the bench.

[32] Johnson is highly involved in community work, and regularly hosts back to school programs, visits local hospitals and pays for low income youth to attend games.

Shortly after the release of Drake's Nothing Was the Same album, Johnson bought all the copies from two local stores and handed them out to random pedestrians in Toronto.