Thibault, 75, served as Quebec’s representative to the Queen for 10 years between 1997 and 2007.
Allegations that Thibault spent $700,000 of taxpayers’ money on parties and trips unrelated to her official duties as lieutenant-governor first came to light in 2007.
Thibault denied the allegations, but an auditor general’s report later supported them.
An inquiry by Quebec’s National Assembly then followed, as did charges of fraud, breach of trust and forgery by police investigators.
The path to trial was delayed numerous times as Thibault appealed the charges.
Thibault claimed immunity to prosecution because she was a representative of the Queen at the time, an argument the Supreme Court of Canada ruled against last year.
On Tuesday, the Crown began calling the first of an estimated 40 witnesses.