The defendants, Elisabetta Grillo, 41, and her sister Francesca, 36, were accused of charging $1.1 million on household credit cards without permission, the Guardian reported.
The sisters were also accused of withdrawing $243,000 for their personal use.
The pair began working for the couple at the start of their relationship in 2001, and were fired over the spending when they divorced.
The sisters argued that Lawson, a TV cook, was a habitual cocaine user and that she allowed them to use the credit cards.
During the trial, Lawson admitted that she had taken cocaine seven times, but only once since her first husband died in 2001.
She also admitted to smoking marijuana during the last year of her marriage to Saatchi.
Attorneys representing the personal assistants claimed that Lawson downplayed her drug use out of a desire to break into the American TV market.
The jury delivered the verdict Friday after eight hours and 52 minutes of deliberations, the Guardian said.