After the ruling, a calm but defiant Del Mastro insisted that he would continue to sit in the House of Commons - even though anyone convicted of breaking the Canada Elections Act is barred from doing so for at least five years.
"This is obviously not a final decision," said Del Mastro, a former parliamentary secretary to Prime Minister Stephen Harper whose role during question period was to defend the government against claims of dirty electoral tricks.
"I've always maintained and I maintain ardently that I've in no way broken any of the laws governing elections. I would suggest we're going to take a very hard look at this ruling and we'll come up with a plan going forward."
The ruling itself was sharp-edged and scathing at times, casting doubts on Del Mastro's own credibility as it convicted him of exceeding spending limits, failing to report a personal contribution of $21,000 to his own campaign and knowingly submitting a falsified document.
He was found guilty on a fourth charge as well, but that charge was stayed.
"The defence evidence does not raise a reasonable doubt about the allegations; further, the evidence I accept does prove the case beyond a reasonable doubt," said Ontario Court Justice Lisa Cameron, as the accused sat stone-faced, his hands in his lap.
"There are a number of inconsistencies and improbabilities," she said of his testimony. "At times, the way in which he testified led me to believe that he is avoiding the truth ... On a number of occasions Del Mastro did not answer the questions put to him in cross examination, he frequently obfuscated the evidence."
Del Mastro faces a maximum penalty of one year in jail and a $2,000 fine on each charge. A sentencing hearing has been scheduled for Nov. 21.
Much of the case pitted Del Mastro's testimony against that of Frank Hall, the president of a now-defunct data consulting firm called Holinshed, whose alleged services lay at the centre of the case.
Del Mastro's defence, Cameron noted, took the position that Hall was lying to the court about the nature of the work he had done for the Peterborough MP during the 2008 campaign period.
While she found that Hall, who was alleged to have altered dates on contracts for voter identification calls, "essentially turned a blind eye to the machinations" of Del Mastro, she nonetheless accepted his evidence to the court.
"I would not characterize Hall's evidence as evasive, although it was at time cautious or careful," she said. "It's clear from the evidence that Holinshed was doing work for the Del Mastro campaign during the election period, specifically voter ID and (get-out-the-vote) calling."
Crown lawyer Tom Lemon said afterward that he will "seriously consider" whether to ask for jail time for Del Mastro.
"In our view, it was an overwhelming case," Lemon said outside court. "We'll be considering what position we would take and whether to ask for custodial sentence."
Del Mastro's lawyer Jeff Ayotte, on the other hand, said he didn't expect his client to end up behind bars.
"It would be very surprising to me if Mr. Del Mastro received a period of incarceration," Ayotte said. "It would be precedent-setting."
Del Mastro's future in the House of Commons, where he sits as an Independent, appeared equally fuzzy in the immediate aftermath of the decision.
Under the Canada Elections Act, anyone convicted of an offence considered illegal under the act is not entitled to sit in the House of Commons for five years from the date of conviction. It's not clear what happens when such a conviction is under appeal.
"This has been part of a pattern of behaviour that we've seen from this government," said NDP House leader Peter Julian as he asked the Speaker for clarity on whether Del Mastro should be allowed to keep his seat.
"The decision to remove a sitting member is one for the House ... only the House has the inherent right to decide matters affecting its own membership."
In a statement, government House leader Peter Van Loan later acknowledged the "serious concerns" raised by the decision and recommended referring the matter to the procedure and house affairs committee.
During the trial, Del Mastro denied Hall's claim that Holinshed provided hundreds of hours of voter-ID calling for his 2008 campaign - a claim that was supported by emails between Hall and Del Mastro that were found on Hall's computer by investigators.
Del Mastro testified that he had rebuffed Hall's efforts to get him to buy Holinshed's voter ID services, and denied ever receiving or sending the emails in question.
Cameron singled out the issue of electronic evidence, which Del Mastro's defence had worked hard to discredit.
"There was certainly no evidence of tampering with the e-data," she said. "There seems to be no air of reality to the suggestion that that has happened." ILLUS: Dean Del Mastro smiles outside a courthouse in Peterborough in June 2014.