Stevie has competed in 27 Challenger tournaments in his career for singles in the United States, Turkey, Canada, and France. He won his first challenger tournament in the Comerica Bank Challenger played in Aptos, California. He won it in the summer of 2012, before the 2012 U.S. Open. In the finals, he defeated Robert Farah in straight sets, 6–3, 6–3, gaining 100 points, as well as Farah is his good friend and former Trojan teammate where they played doubles together and won 2 NCAA Championships for USC. A month after his win in Aptos, Stevie competed in 2 challenger tournaments in Turkey and France. In Izmir, Turkey, Stevie reached the semifinals and won three matches to get there. In Orléans, France, Stevie reached the second round and lost to the no.2 seed David Goffin of Belgium. However, the match was tightly contested as Goffin squeezed out a win and won 7–5, 6–4. A couple of weeks later in the 2012 Tiburon Challenger, Stevie was ousted in the semifinals by Jack Sock 4–6, 6–7(4). Next, Johnson competed in the 2012 Charlottesville Challenger but fell to Rhyne Williams in the Round of 16. Johnson planned on playing in the Knoxville Challenger as well as the JSM Challenger of Champaign–Urbana to finish the year, but a shoulder injury forced him to pull out. The shoulder injury also forced him to miss the Australian Open Wild Card Playoffs. Stevie has compiled an overall record of 32 wins and 18 losses on the Challenger Tour and has earned In doubles, Stevie has had equal success on the Challenger Tour compiling an overall record of 13 wins and 8 losses including a title in Knoxville, Tennessee with Austin Krajicek in 2011. He also made it to the finals in the 2011 Tiburon challenger partnered with Sam Querrey, but they lost 6–10 in the 3rd set super tie-breaker. In 2012, Stevie reached the semifinals in Tiburon partnered with Robert Farah, as the #1 seeds. Johnson played doubles in the 2013 Maui Challenger being seeded no.2 and reached the semifinals as his partner was Alex Bogomolov, Jr. Johnson played singles and doubles in the 2013 Sarasota Open. In singles, he lost in the quarter-finals to the eventual champion, Alex Kuznetsov, 2–6, 6–3, 1–6. In doubles, he partnered with Bradley Klahn, and they won three matches to reach the finals but lost 7–6(5), 6–7(3), 9–11. Johnson played three more clay challengers before the French Open and lost in the first round in each. After a successful French Open, Johnson won his second career challenger at the Aegon Nottingham Challenge defeating Ruben Bemelmans in the finals. Winning this tournament, helped grant him a wild-card into Wimbledon. Johnson did not finish the year strong on the Challenger Tour as he went 1-5 to finish the season. In his second challenger event of the season, Johnson won the 2014 Challenger of Dallas only dropping one set throughout the tournament. He defeated fellow American Ryan Harrison along the way, and he defeated Tunisian Malek Jaziri in the finals. After the match, he stated,"I was struggling with confidence a little before the start to this year and to come out and win the tournament here makes it more special." One month later in the 2014 Irving Tennis Classic, Johnson beat three top-100 players along the way to reach the finals, where he lost to Lukáš Rosol. A win at the 2014 Open Guadeloupe Challenger Tour tournament boosted Johnson's singles ranking to a career-high 69, and gave him his fourth career challenger title. After taking a month off from competing in tournaments, Johnson's next challenger tournament was the BNP Paribas Primrose Bordeaux where he was the number two seed. He lost in the finals to number one seed Julien Benneteau. Johnson kicked off his grass court season as the number two seed in the 2014 Aegon Trophy where fell in the quarterfinals to Gilles Müller.