A majority of the justices on the bench found Joao Paulo Cunha, a top politician in Lula’s Workers’ Party (PT) who headed the Chamber of Deputies, guilty of bribery, embezzlement of public funds and money laundering.
Thirty-eight former ministers, lawmakers, businessmen and bankers are facing prosecution before the Supreme Court over the alleged scheme that ran from 2002 to 2005 during Lula’s first term.
Cunha, who is running for mayor in a city in the Sao Paulo region, is accused of having received 50,000 reals (about US$25,000) from an advertising agency with alleged funding ties to the PT.
The former director of marketing at the public-owned Banco do Brasil was also convicted on charges of embezzling US$37 million.
The justices acquitted former communications minister Luiz Gushiken, saying there was insufficient proof to find him guilty.
In the coming weeks, the court is due to rule on the cases of former chief of staff Jose Dirceu and Delubio Soares de Castro, former PT treasurer.
The so-called Mensalao Scandal nearly cost Lula his re-election in 2006, even though he has been cleared of involvement.
According to charges that first surfaced in 2005, PT members allegedly offered bribes to members of Congress in exchange for their votes.
Prosecutors allege that the bribe money was skimmed from the advertising budgets of state-owned companies through a firm owned by businessman Marcos Valerio de Souza, one of the accused.
Lula, who is recovering from throat cancer, has said he was betrayed and has offered public apologies on behalf of the PT, which he founded. The party denied any vote-buying ahead of the trial.