Williamson was originally charged with 23 federal counts.
Among the charges, she was accused of co-conspiring in a scheme to steal about $225,000 from a dormant campaign account belonging to Xavier Becerra.
At the time, he was serving as the US Health and Human Services Secretary under President Biden.
Williamson faces a maximum sentence that includes up to 30 years in prison on the conspiracy charge, including a fine of $1 million.
She served as Newsom's chief of staff for about two years and left in November 2024.