Mäkinen won the Group N Finnish Rally Championship in 1988 driving a Lancia Delta HF 4WD. His first WRC victory came on the 1994 1000 Lakes Rally (now Rally Finland) in a Ford Escort RS Cosworth. By the standards of the sport he was a late developer, only securing his first full-time manufacturer seat in 1995 — in a Group A Mitsubishi Lancer Evolution alongside Kenneth Eriksson.
A Safari Rally win in 1996 gave Mäkinen the platform to build a dominant championship lead, which he sealed in Australia, edging out Subaru's Colin McRae. He went on to win the WRC drivers' title every year from 1996 through 1999, all behind the wheel of the Ralliart Mitsubishi Lancer Evolution. His navigators during this period included Seppo Harjanne, Kaj Lindström, and Risto Mannisenmäki.
In 1998, Mitsubishi also captured the constructors' title, the team's sole manufacturers' championship, with Mäkinen and young Briton Richard Burns among its drivers. To mark Mäkinen's string of successes, Mitsubishi produced a road-legal Lancer Evolution VI "Tommi Mäkinen Edition" in 2000, distinguishable by its unique front bumper and, on some examples, a red-and-white livery echoing his rally car.
Mäkinen opened the 2000 season with a Monte Carlo Rally victory but ultimately relinquished his championship reign to fellow Finn Marcus Grönholm. He remained with Mitsubishi through the end of 2001. A crash on the Corsica round that year broke co-driver Mannisenmäki's back and effectively ended Mäkinen's top-line career.
He moved to the Prodrive-run Subaru World Rally Team for 2002 and claimed one final career victory at the 2002 Monte Carlo Rally, after the on-the-road winner Sébastien Loeb was excluded for a technical infringement. Mäkinen retired from the WRC after the 2003 season, concluding his career with a third-place finish at Rally Great Britain. He ranked fifth all-time in WRC wins (24) at the time of his retirement and tied with Juha Kankkunen for third in championships (4).
In 2004, Mäkinen established Tommi Mäkinen Racing Oy Ltd, a company dedicated to preparing rally cars and supporting drivers. In 2016, he was appointed team principal of Toyota Gazoo Racing WRT, Toyota's factory WRC entry. Under his leadership, Toyota won the 2018 manufacturers' championship — the manufacturer's first since 1999 — fulfilling the historic double of championship success as both driver and team principal. Mäkinen departed the role at the end of 2020 to become Toyota's motorsport advisor.
Mäkinen's four straight WRC titles remain one of the most sustained dominant runs in rally history. His era with the Mitsubishi Lancer Evolution cemented both driver and car as icons of 1990s rallying. The licensed Tommi Mäkinen Rally video game, released in 1998, further extended his cultural footprint during the height of his success. He also won the 2000 Race of Champions, and resides between Jyväskylä and Monte Carlo, Monaco.