Mikhailov, who was an officer of the Soviet special police OMON, was convicted for killing Lithuanian border guards and police officers who were guarding Lithuania's border soon after it declared independence from the USSR. The Court of Appeals has also reclassified the offence as crime against humanity.
The sentence came into force after the Court of Appeal's ruling.
The Prosecutor General's Office had asked the court to reclassify the criminal act committed by Mikhailov from murder to crime against humanity. The latter is defined in Lithuania's law as killing conducted by supporting the policy of a foreign state and organizations of a foreign state and by attacking, killing and injuring civilians in a systematic way and on a large scale.
"The convict was among the persons who performed acts of violence against the eight officers," Judge Aloyzas Kruopys said.
Latvian citizen Mikhailov was detained by Latvian law-enforcement bodies on November 28, 2007 under a European arrest warrant issued by Lithuania. He has been held in detention at Lukiškės Prison in Vilnius since January 2008.
It emerged later that Mikhailov also has Russian citizenship.
The Vilnius Regional Court had ruled that Mikhailov, as a member of the Soviet Interior Ministry's special police unit OMON and acting as part of a squad organized by Cheslav Mlynik, together with other members of the squad, Andrei Laktionov and Alexander Ryzhov, intentionally killed Lithuanian customs and police officers Antanas Musteikis, Stanislavas Orlavičius, Ričardas Rabavičius, Algirdas Kazlauskas, Juozas Janonis, Algimantas Juozakas and Mindaugas Balavakas, and attempted to kill Tomas Šernas.