Tomashevsky was born in Saratov on 1 July 1987.[1][2] He won the Russian under-10 championship in 1997 and the Russian under-18 championship in 2001, at the age of 13 years,[3] in Rybinsk with a score of 9½ points from 11 games.[4] In 2004 he finished runner-up in the U18 division of the World Youth Chess Championships.[5]
In 2007, he came second in the Aeroflot Open.[6] In 2009, Tomashevsky won the 10th European Individual Chess Championship after tie-breaks. The decisive match against Vladimir Malakhov went into armageddon stadium, where Malakhov blundered a rook in a winning position.[7] In January 2010, he played for the gold medal-winning Russian team at the World Team Chess Championship 2009 in Bursa.[8]
In 2011, he tied for first place with Nikita Vitiugov and Lê Quang Liêm in the Aeroflot Open, placing third on a tiebreak.[9] He was one of the seconds to Boris Gelfand for the World Chess Championship 2012.[10]
In February 2015, Tomashevsky took clear first place in the Tbilisi leg of the FIDE Grand Prix 2014–15 series scoring 8/11, 1½ points ahead of second-placed Dmitry Jakovenko, with no losses and wins over Baadur Jobava, Alexander Grischuk, Shakhriyar Mamedyarov, Maxime Vachier-Lagrave and Rustam Kasimdzhanov.[11] His performance rating in this tournament was 2916.[12] In August 2015, he won the Russian Championship Superfinal in Chita, Zabaykalsky Krai with 7½/11.[13] The following year, he played for bronze medal-winning team Russia in the 42nd Chess Olympiad in Baku. In 2019 Tomashevsky won his second Russian Championship in Votkinsk – Izhevsk, Udmurtia with a score of 7/12.[14]
Partly for being a mostly positional player, partly for wearing glasses and being well-educated, Tomashevsky earned himself the nickname "Professor" among the chessplayers.[3]
In 2025, it was reported that Tomashevsky was the leader of a team of seven grandmaster coaches that the Russian Chess Federation had assigned to the ten-year-old chess prodigy Roman Shogdzhiev.[15][16]
