Sergachev began his junior career in Russia, where he played for Irbis Kazan of the Molodezhnaya Hokkeinaya Liga (MHL).

[10] On 6 October 2017, Sergachev made his Lightning debut, where he also recorded his first career NHL assist and point on a goal by Nikita Kucherov in a 5–3 victory over the visiting Florida Panthers.

[11] On 19 October, Sergachev scored his first and second career NHL goals against the Columbus Blue Jackets at Nationwide Arena, which came in a 2–0 Lightning victory.

[18] On 21 April 2018, Sergachev recorded his first career playoff goal in a 3–1 series clinching win over the New Jersey Devils at Amalie Arena.

[19] On 13 January 2019, the NHL Department of Player Safety fined Sergachev $2,403.67 for cross-checking Johan Larsson during the game against the Buffalo Sabres the night before at KeyBank Center.

[21] Sergachev joined Eric Brewer and Victor Hedman as the only Lightning defensemen to record three points in a single playoff game.