Chagin first competed at the Dakar Rally in 1990. He did not figure prominently in the overall standings until the late 1990s, finishing 30th in 1996 before failing to finish in 1998. His first Dakar victory came in 2000, when he won four stages. He did not finish in 2001 despite taking five stage wins. Victory followed again in 2002 with seven stage wins, and in 2003 with two stage wins, and in 2004 with four. A non-finish in 2007 interrupted a sequence broken only by 2005 (18th overall, six stages) and the 2008 edition — which was cancelled due to security concerns and replaced by the Central Europe Rally. He was second in 2009, winning four stages.
His 2010 performance is regarded as the defining effort of his career. Chagin won nine of the fourteen truck stages available at that year's Dakar Rally, which had moved to South America, setting the record for the most stage wins by a truck driver in a single edition. He also won the overall event. In 2011 he won a seventh time, taking seven stages.
Chagin's 63 total Dakar stage wins across his career placed him second in the all-time list across all categories at the time of his retirement, five behind Stéphane Peterhansel. His seven Dakar titles in the truck category surpass the record previously held by Karel Loprais by one.
Beyond the Dakar, Chagin built one of the most comprehensive records in rally raid competition. He won the Abu Dhabi Desert Challenge six times (1999, 2001, 2002, 2003, 2004, 2005) and the Master-Rally four times (1995, 1996, 2000, 2002). His other event victories include the Optic 2000 Tunisia Rally (2000, 2001), Rally Orientale-Kappadokia (2003, 2004), Khazar Steppes (2004, 2006), Baja Italia (2000), Baja Pearl (2000), Kalmykia (2000), and The Quiet Don (2003). He also finished on the podium at the Desert Challenge in 2000 and at the Baikonur–Moscow rally in 1997.
After retiring from competitive driving, Chagin moved into a team management role within the Kamaz-Master organisation, acting as team leader for the squad that had defined his own career.
Chagin received multiple state honours from the Russian Federation, including the Order of Merit for the Fatherland (4th class), the Order of Courage, the Order of Honour, the Order of Friendship, and the Order of the Badge of Honour of the Soviet Union. He was also awarded the Medal "In Commemoration of the 1000th Anniversary of Kazan" and the Medal "For Valiant Labour" from the Republic of Tatarstan.