ATHENS — The former mayor of Thessaloniki, Greece’s second-largest city, and two of his top aides were sentenced to life in prison Wednesday after being found guilty of embezzling almost $23.5 million in state funds, a rare conviction in a case involving the political corruption that has contributed to the country’s dysfunction and economic decline.

A court in the northern port city found that the local authorities had set up an ‘‘embezzlement machine,’’ and that Vassilis Papageorgopoulos, a prominent conservative who served two terms as mayor from 1999 to 2010, had been aware of the whole plan but had stayed on the sidelines, feigning ignorance. The scheme was conceived by Michalis Lemousias, a general secretary of the city administration, who operated with Panagiotis Saxonis, the city’s treasurer, the court found.