Everts grew up in Neeroeteren, a sub-municipality of Maaseik in the Belgian province of Limburg, in a household steeped in motocross. His father Harry won four world championships between 1975 and 1981, and Stefan began riding motorcycles at the age of four. The family legacy gave Everts early technical grounding, though the heights he eventually reached far exceeded what any other motocross rider — including his father — had achieved.
At seventeen he made his debut in the 125cc World Championship, and just two years later secured his first world title in that class, becoming the youngest world champion in motocross at the time. The progression from 125cc to 250cc and eventually to the premier 500cc and MX1 classes was steady and relentless, and Everts gathered titles across all of them.
Everts' championship career spanned six manufacturers and four displacement classes, a breadth of success unmatched in the sport. His first title came in the 125cc class in 1991 aboard a Suzuki. He then moved to 250cc and claimed three consecutive world championships in 1995, 1996, and 1997, riding first for Kawasaki and then Honda. He added further 250cc titles in subsequent seasons and eventually stepped up to the 500cc and Motocross GP (MX1) categories, where he won five more championships between 2001 and 2006.
One of Everts' most notable statistical distinctions is that he became the only rider in history to win world titles on all four major Japanese manufacturers — Suzuki, Kawasaki, Honda, and Yamaha. He also became only the second rider after Eric Geboers to earn the informal designation of "Mr. 875cc" by winning championships in the 125cc, 250cc, and 500cc classes.
His final season, 2006, was extraordinary in its dominance: he won fourteen of fifteen Grands Prix on the MX1 calendar aboard a Yamaha, a win-to-race ratio that underscored just how complete a racer he had become. That year he took his tenth and final world championship.
His championship roll:
1991: World Champion, 125cc (Suzuki)
1995: World Champion, 250cc (Kawasaki)
1996: World Champion, 250cc (Honda)
1997: World Champion, 250cc (Honda)
2001: World Champion, 500cc (Yamaha)
2002: World Champion, 500cc (Yamaha)
2003: World Champion, Motocross GP (Yamaha)
2004: World Champion, Motocross GP (Yamaha)
2005: World Champion, MX1-GP (Yamaha)
2006: World Champion, MX1-GP (Yamaha)
He also contributed to Belgium's Motocross of Nations victories in 1995, 1997, 1998, 2003, and 2004.
Everts was widely admired for a riding style that looked almost effortless, characterised by a notably upright standing position even through tight corners. Unlike many contemporaries who revved their engines hard through each gear, he often chose a higher gear and used engine torque to carry speed through corners — a technique effective on both the two-stroke 125cc and 250cc machines of his earlier career and the four-stroke machinery that followed. His consistency in the later stages of his career was exceptional; the fourteen-to-one win-loss record in 2006 was not an anomaly but a reflection of accumulated mastery.
After retiring from active competition at the end of 2006, Everts became race director and coach for the KTM factory motocross team. In that role he was responsible for the technical, mental, and physical preparation of KTM's works riders. He renewed his contract with the Austrian manufacturer in July 2007 for a further two years. He settled in Monaco after his retirement from racing.
In late 2018 Everts suffered a life-threatening malaria infection after being bitten by a mosquito during a charity motocross event in Lubumbashi, in the Democratic Republic of the Congo. He was admitted to the intensive care unit of UZ Leuven in early December 2018 in critical condition and placed in an induced coma. He woke from the coma on 17 December 2018. The illness and subsequent surgeries resulted in the loss of eight toes, though he recovered the ability to walk. His treating physicians at UZ Leuven described his survival as having been in serious doubt.
Everts received numerous honours across his career. He was named Belgian Sportsman of the Year five times — in 2001, 2002, 2003, 2004, and 2006 — a total only surpassed in Belgian sports history by Eddy Merckx. He received the Belgian National Sports Merit Award in 2003 and a Lifetime Achievement award in 2006. A statue was erected in his honour in Neeroeteren in 2006. In 2011 he received the USGP Legends and Heroes Award. The Flemish Sportsjewel (Vlaams Sportjuweel) was awarded to him in 1997.