2006–07 season, Johnson quickly established a top defensive role and accumulated 4 goals and 24 points in 41 contests to lead all WCHA rookie defenseman and earn a selection to the WCHA All-Rookie Team. Johnson concluded his collegiate career after his freshman year when he was signed by the St. Louis Blues to a three-year, entry-level contract on April 20, 2007. Professional St. Louis Blues (2007–2011) Upon concluding his collegiate career, Johnson attended the Blues' Development Camp and 2007 Prospects Tournament. His first career NHL goal came on October 6 in a 5–3 win over the Los Angeles Kings. The goal was scored on a power play with 7:17 left in the third period and became the game-winning goal. After playing three games with the Blues, Johnson suffered a slight bone fracture in his foot. As a result, he was re-assigned to the Peoria Rivermen of the American Hockey League (AHL) on a conditioning stint. Johnson re-joined the Blues on November 7 but struggled to get back into the lineup. However, once he became a mainstay in the Blues' lineup he began a productive rookie campaign. By January, he tallied 16 points to tie for 10th amongst all NHL rookies and fourth on the team with a plus-10 rating. As such, he was selected for the NHL's 2008 YoungStars Game. As the season continued, his ice time fluctuated from a season high 22:0 to a season-low 11 minutes although he typically averaged 17 minutes per game. He finished his rookie season averaging over 18 minutes of ice-time per game to go with 5 goals and 28 assists in 69 games. Following his rookie season, Johnson injured his right knee during the 2007 offseason which sidelined him for the first three days of training camp. It was later confirmed by the team's orthopedic surgeon that Johnson had torn his ACL and suffered a tear to his MCL. In November, Johnson underwent surgery successful right knee ACL surgery. After missing the 2008-09 season with a knee injury, Johnson returned for the 2009–10 season strong by setting new career-highs with 10 goals, 29 assists for 39 points. On August 2, 2010, as a restricted free agent, Johnson re-signed with the Blues with a two-year, $5.2 million contract. In the first year of his newest contract, Johnson was named an assistant captain alongside David Backes, Barret Jackman, and Alexander Steen. Johnson tallied one goal