Avdeev was born in Chapayevsk, Samara Oblast (formerly Kuybyshev Oblast) on 1 January 1956, Russian SFSR. He graduated from Moscow Physics-Engineering Institute in 1979 as an engineer-physicist. From 1979 to 1987 he worked as an engineer for NPO Energiya. He was selected as a cosmonaut as part of the Energia Engineer Group 9 on 26 March 1987. His basic cosmonaut training was from December 1987 through to July 1989. He retired as a cosmonaut on 14 February 2003.

Avdeev at one point held the record for cumulative time spent in space with 747.59 days in earth orbit, accumulated through three tours of duty aboard the Mir Space Station. He has orbited the earth an incredible 11,968 times travelling about 515,000,000 kilometres. In August 2005, this record was taken by another cosmonaut, Sergei K. Krikalev.

Avdeev is married with two children. He is an amateur radio operator, and his call sign is RV3DW.

Time traveller
According to several reputable sources, Sergei holds the current known record for time travel by a human being. In his over 748 days aboard Mir (cumulative across three missions) he went approximately 17,000 mph and travelled roughly 0.02 seconds (20 milliseconds) into the future. A common misconception is that the Apollo astronauts hold the record -- they did go faster than Avdeev, but they were only in space for a few days.