Tänak won the Estonian Rally Championship in 2008 and 2009 while driving for a team run by former world rally winner Markko Märtin. He made his WRC debut at the 2009 Rally de Portugal, finishing 20th, and won the European Pirelli Star Driver shootout in September 2009, earning a six-round WRC programme in 2010. He won the PWRC category at the 2010 Rally Finland and 2010 Rally GB.
In 2011 he drove for MM-Motorsport under Märtin's direction before joining M-Sport Ford World Rally Team as their number-one driver for the full 2012 season. Despite showing raw pace throughout 2012, including a stage win at Rally Sweden and a third-place finish in Italy, crashes and retirements limited his championship tally to eighth overall. A difficult 2012 led to his departure from M-Sport, and 2013 was spent competing in Estonian national rallies under his own OT Racing team.
Tänak returned to the WRC with the DMACK World Rally Team in 2014 and rejoined M-Sport Ford for the 2015 and 2016 seasons, recording podiums but unable to convert pace into consistent results. The inflection point came in 2017 when Tänak returned to M-Sport alongside Sébastien Ogier, changed co-drivers by bringing in Martin Järveoja, and immediately won in Sardinia after inheriting the lead from Hayden Paddon. He followed with another victory in Germany, finishing third in the championship.
Before the 2018 season Tänak signed with Toyota Gazoo Racing. He adapted quickly, winning in Argentina, then taking three consecutive victories in Finland, Germany, and Turkey, finishing third in the championship behind Ogier and Thierry Neuville.
In 2019 Tänak delivered the most dominant campaign of his career. He led the championship from an early stage, winning in Sweden, Chile, and four additional rounds, before sealing his maiden World Rally Championship at the Rally de Catalunya when early power-steering failure dropped Ogier out of contention. Tänak's six victories and championship margin made it a definitive title. He became the first non-Frenchman to win the drivers' championship since 2003 and the first Toyota driver to win the title since Didier Auriol in 1994.
In October 2019 Tänak signed a two-year deal with Hyundai. His title defence began with a dramatic crash at the 2020 Monte Carlo Rally when his Hyundai i20 bottomed out on a bump and flew off a forty-metre cliff at 180 km/h, rolling through trees before landing on the road below. Both Tänak and Järveoja walked away uninjured.
Through 2020 and 2021 reliability issues and accidents limited Hyundai's championship challenge, though Tänak won the 2021 Arctic Rally Finland. In 2022, after new hybrid-mandated Rally1 regulations came into force, Tänak finished as championship runner-up behind runaway champion Kalle Rovanperä, winning in Sardinia and Finland. He finished third in the championship standings on four separate occasions across his career.
Tänak rejoined M-Sport Ford for 2023 with the Ford Puma Rally1 car, winning in Sweden and Chile before reliability issues undermined his championship prospects. He returned to Hyundai for 2024, finishing second in Sardinia in one of the season's closest finishes — beating Sébastien Ogier by just 0.2 seconds — and winning the Central European Rally. A deer strike in Poland and a roll in Finland cost him championship points, and the title went to Neuville.
In 2025, Tänak mounted a late-season charge against championship leader Elfyn Evans before engine problems in Chile ended his challenge. He won in Greece and remained competitive on gravel events before announcing at the conclusion of the 2025 Rally Japan that he would step down from full-time WRC competition.
Tänak married Janika Tänak in 2016, and they have two children. He was named Estonian Athlete of the Year in 2017, and he and Järveoja won the Estonian Sports Team of the Year award in 2018. A documentary, Ott Tänak: The Movie, released theatrically in Estonia on 11 April 2019, chronicled his early life on the island of Saaremaa and his path to the WRC.
With twenty-two WRC victories, Tänak placed eighth on the all-time wins list. His 2019 title broke a French monopoly on the WRC championship that had persisted for sixteen years and established Estonia as a significant nation in world rallying.