At USC, Johnson was coached by Peter Smith and majored in Human Performance, but left when he was three classes short of attaining his degree.

In July 2012, Johnson signed a clothing deal with Asics America and is represented by Sam Duvall at Lagardere Unlimited.

Some of Johnson's junior accomplishments include being the 2008 Kalamazoo Doubles finalist and winning the 2008 Southern California Sectional Boys 18 championship in straight sets over JT Sundling.

He won CIF singles championships in 2006 and 2007 and was named the Orange County and Los Angeles Player of the Year in both seasons.

He missed just two because of junior tennis events and did not lose a set in team competition on his way to winning his second consecutive Golden West League title.

His only high school loss of 2006 was a three-set defeat in the semifinals of the Ojai Tennis Tournament to eventual champion Jason Jung of West Torrance.

As a sophomore, he was selected to represent the United States in the fourth annual Master'U BNP Paribas, an intercollegiate competition in which eight countries from around the world play for the title.

In his senior season, he captured the 2011–2012 NCAA Singles Championship, defeating Kentucky's Eric Quigley in the final, overcoming a strained abdomen and shin splints and a bout of food poisoning to retain his title.

In his college career, he became a seven-time Intercollegiate Tennis Association (ITA) All-American, two-time NCAA Singles Champion, and he captured the team title for the Trojans in all four of his years there.

In the finals, he defeated Robert Farah in straight sets, 6–3, 6–3, gaining 100 points, as well as A month after his win in Aptos, Stevie competed in 2 challenger tournaments in Turkey and France.

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.

Shortly after his college season, Johnson received a wildcard into the 2011 Farmers Classic, where he lost in the first round to Gilles Müller in three sets.

Johnson registered his first ATP win in a main draw against Donald Young in the 2012 BB&T Atlanta Open before losing to Sock in the second round.

Johnson went to the 2013 French Open, qualifying for the first time[10] and made it through to the main draw before losing in the first round to Albert Montañés, who had just won Nice[11] the previous week.

Receiving a wildcard into the maindraw of Wimbledon, Johnson lost a tight first round match to fellow American Bobby Reynolds.

Johnson received a wildcard into the maindraw of the 2014 BNP Paribas Open, but fell to the red-hot Spaniard Roberto Bautista Agut in straight sets, who knocked out #4 seed Tomáš Berdych in the next round.

Johnson registered his first grass court ATP win at the 2014 Gerry Weber Open when he defeated Frenchman Albano Olivetti.

Returning to the U.S., Johnson competed in the 2014 Hall of Fame Tennis Championships, where he lost in the quarterfinals to eventual champion Lleyton Hewitt.

At the 2015 US Open, Johnson reached the semifinals in doubles for the first time at a Major partnering Sam Querrey after defeating en route the top seeded pair of the Bryan brothers in the first round.

He won his first ATP Tour level title at the 2016 Aegon Open in Nottingham, UK, defeating Pablo Cuevas in the final.

After reaching three straight quarterfinals in his next three tournaments, Johnson lost to Roger Federer at Indian Wells and had a disappointing second-round loss at Miami to Nicolas Mahut.

Johnson won his third title in Houston on clay courts after defeating five Americans: Ernesto Escobedo, Frances Tiafoe, John Isner, Taylor Fritz, and Tennys Sandgren.

In July, Johnson won the grass-court Hall of Fame Championship in Newport, Rhode Island, defeating Indian Ramkumar Ramanathan over three sets in the final.

In March, Johnson beat Taylor Fritz in the first round of Indian Wells, but fell to young Canadian Denis Shapovalov in the second.

When tennis returned in August Johnson, partnering Austin Krajicek as a wildcard pair, reached the semifinals at the Western & Southern Open, held in New York.

Partnering with Austin Krajicek as wildcard pair, he reached his maiden Masters 1000 final at the Western & Southern Open in Cincinnati defeating No.

At the 2022 Wimbledon Championships he reached the third round for a fourth time in his career defeating Grigor Dimitrov by retirement and wildcard Ryan Peniston before losing to another Brit ninth seed Cameron Norrie.

4 spot on the top plays of SportsCenter that evening against Alex Bogomolov Jr. of the Texas Wild with an amazing rally that ended with Johnson slipping and sliding for a volley winner.

Steve made a successful debut with the Breakers in his first season, leading him to be named the Mylan WTT Male Rookie of the Year.

Johnson played five seasons with World TeamTennis starting in 2013 when he made his debut with the Orange County Breakers, earning the Male Rookie of the Year award.