Tetsuya Shiroo, 60, who was associated with Japan's largest criminal syndicate, was convicted of shooting Mayor Iccho Ito as he campaigned for re-election in April last year.
He said Shiroo plotted the killing because he blamed his financial problems on the mayor's administration in the southwestern city.
Shiroo had previously had a dispute with the city administration over compensation for a traffic accident.
The accused "tried to carry out his grudge by thwarting the former mayor's campaign for a fourth term.
Shiroo said during the trial that he was ready to be hanged, according to Jiji Press.
"I would like to sincerely receive the ultimate punishment," he was quoted as saying.
The country has seen several attacks on politicians in recent years. In 2002, an ultra-nationalist stabbed to death Koki Ishii, an opposition lawmaker known for his aggressive investigations of corruption.