[3] After agreeing to a contract with the Knights, Zadorov began his major junior career in the Ontario Hockey League (OHL) in the 2012–13 season, joining fellow Russian Vladislav Namestnikov.

[4] Earning a roster spot in the 2013 CHL Top Prospects Game, Zadorov helped the Knights claim the OHL championship to advance to the Memorial Cup.

[3] As he was selected to the OHL First-All Rookie Team, he scored the game-winner in the opening game against the Western Hockey League's Saskatoon Blades before losing in the semi-finals.

[6] Zadorov attended his first rookie and NHL training camp and after his third pre-season contest he was signed by the Sabres to a three-year, entry-level contract on 26 September 2013.

[13] Despite a benching during the rookie pre-season tournament in Traverse City, Michigan, Zadorov rebounded to have a strong training camp to make the Sabres' opening-night roster in the 2014–15 season as a permanent addition to the blueline.

[14][15] He initially only saw limited playing time as a rotational option to the third-pairing under Ted Nolan and was primarily on the team for practice only (he was not yet eligible for the American Hockey League and a tie-up involving Zadorov's KHL rights prevented him from being returned to juniors).

[17][18] In developing a shut-down role through his physicality, Zadorov showed offensive glimpses, collecting his first multi-point game in scoring an overtime marker in a 4–3 victory over the Florida Panthers on 13 December 2014.

"[24] He began the season with the team but was reassigned to their American Hockey League (AHL) affiliate, the San Antonio Rampage, in order to improve his game.

[34] He ended his fourth year campaign playing in 70 regular season games and recording seven goals, seven assists, and accumulating 75 penalty minutes.

[36] He was listed as day-to-day with a lower-body injury before fully returning to team practice on 24 September, with fellow injured players Pierre-Édouard Bellemare and Erik Johnson.

[40] During their Second Round contests against the Dallas Stars, Zadorov passed Matt Duchene for 74th place on the franchise playoff points list.

[42] On 10 October 2020, Zadorov's five-year tenure with the Avalanche ended as he was dealt alongside Anton Lindholm to the Chicago Blackhawks in exchange for Brandon Saad and Dennis Gilbert.

On 28 July 2021, Zadorov – an impending restricted free agent – had his negotiation rights traded to the Calgary Flames in exchange for a 2022 third-round pick.

[50] Leaving the Canucks as a free agent having been unable to come to terms on an extension at the conclusion of his contract, Zadorov returned to the Eastern Conference in signing a six-year, $30 million deal with the Boston Bruins on 1 July 2024.

[51] Following a second-round exit with the Avalanche in the 2019 Stanley Cup playoffs, on 11 May 2019, Zadorov was a late inclusion on the Russian squad to make his senior international debut at the 2019 IIHF World Championships held in Bratislava, Slovakia.