Johansson began to play tennis at age five with his father, Krister. In 1989, became European 14s singles champion and won doubles title (with Magnus Norman). Even when he injured his right elbow while playing the Orange Bowl tennis championships 16s in 1991, he reached the final, losing to Spain's Gonzalo Corrales. He finished No. 10 in the 1993 world junior rankings.
That same year he joined the pro tour for the first time. He has managed to win 9 top-level singles titles and 1 doubles title, including the 1999 Canada Masters (df. Yevgeny Kafelnikov) and the 2002 Australian Open Grand Slam championship, which he unexpectedly won (on his 25th attempt at winning a Grand Slam title) after defeating his heavily favored opponent, Marat Safin, in four sets, 3–6 6–4 6–4 7–6. Johansson became the first Swedish player to win a Slam since Stefan Edberg won the 1992 U.S. Open title, and the first Swede to claim the Australian since Mats Wilander in 1988.
Johansson later reached his career-high ranking of World No. 7 in June 2002 and finished the season with a career-best year-end ranking of 14th in the ATP Champions Race, a feat he later repeated in 2005.
A knee injury robbed Johansson of the latter half of the 2002 season and all of 2003, and Johansson was therefore unable to compete on the tour full-time until 2004. In 2005, he became the first Swedish player to reach the semifinals at Wimbledon since Stefan Edberg in 1993, losing to Andy Roddick in a tightly contested four set match, 7–6, 2–6, 6–7, 6–7. At the end of the season, Johansson won his 9th, and until now, last ATP tournament in St. Petersburg, defeating Nicolas Kiefer 6–4, 6–2.
In 2006, the Swede struggled through the season after suffering an eye injury early in the season. The highlights of the season were a 4th round at the Australian Open (where he lost to Ivan Ljubičić), his first doubles title in Båstad, Sweden with countryman Jonas Björkman, and a final in St. Petersburg (lost to Mario Ančić), where he was the defending champion.
At the 2008 Beijing Summer Olympics he reached the doubles final with Simon Aspelin, where he lost against the Swiss team of Roger Federer and Stanislas Wawrinka.
As of 6 March 2009, he has an 18–15 career Davis Cup record (17–12 in singles) in 17 ties, and an 356–292 career overall. He is sponsored by Dunlop Sport for racquets and apparel, and adidas for shoes. He uses a heavily modified Dunlop Pro Revelation racquet 'paintjobbed' to look like the current Dunlop 4D Aerogel 500 racquet.
He announced his retirement in June 2009 after a 16-year career.