Rustam Ibragimov is a former security officer.
The court sentenced the second main defendant, a former official in the Senate administration, Erzhan Utembaev, to 20 years in jail.
Another eight people received prison terms of between three and 20 years.
Kazakhstan introduced a moratorium on executions in December 2003.
The Sarsenbaev murder trial began on June 14 in Taldy-Qorgan, some 250 kilometers north of Almaty.
Sarsenbaev's relatives and supporters believe the assassination in February of the Naghiz Ak-Zhol opposition party leader, his bodyguard, and his driver was commissioned by higher-ranking state officials.
They have called the trial "a farce" and vowed to proceed with their own investigation into the triple murder.
Kazakhstan's Fallen Opposition
February 13, 2006: Altynbek Sarsenbaev -- a leader of the Kazakh opposition, co-chairman of the Naghyz Aq Zhol party, former minister of information, former Kazakh ambassador to Russia, former secretary of Kazakhstan's Security Council -- is found dead with his bodyguard and his driver, shot to death, execution style, with their hands bound behind their backs.
November 11, 2005: Zamanbek Nurkadilov-- prominent Kazakh politician, former mayor of Almaty, former Minister of Extraordinary Situations, and since a vocal critic of President Nursultan Nazarbaev since March 2004 -- is found shot to death, with two shots in his chest and one in his head. Kazakh authorities officially rule the case a suicide.
June 2, 2005: Batyrkhan Darimbet -- opposition journalist and editor in chief of the weekly AZAT -- is killed in what is officially reported as a traffic accident. Relatives and activists assert that it was a political killing.
December 19, 2004: Erzhan Tatishev -- head of Kazakhstan's largest bank, TuranAlemBank -- is killed in what was officially described as a hunting accident. Kazakh political observers allege that it was a premeditated assassination.
July 20, 2004: Askhat Sharipzhan-- independent journalist and political commentator for NAVI online -- dies of injuries sustained several days earlier in what was officially reported as a hit-and-run accident. Relatives and colleagues believe it was an assassination.
November17, 2002: Independent journalist Nuri Muftakh is killed in what is officially reported as a traffic accident. Colleagues and activists regard the death as suspicious.
January 4, 2002: Human rights activist Aleksei Pugaev is found dead, the victim of a hit-and-run car accident. Colleagues regard the circumstances as suspicious and no one is ever arrested in connection with the death.
(compiled by RFE/RL's Kazakh Service)
ARCHIVE To view an archive of RFE/RL's coverage of Kazakhstan, click here.
SUBSCRIBE For weekly news and analysis on all five Central Asian countries by e-mail, subscribe to "RFE/RL Central Asia Report."