James Dorrin Hayward, 55, was found guilty of directing an eight-year-old girl to indecently touch him on two occasions.
The former Nationals upper house member abused the child at two locations before he was elected to office.
The District Court in Perth heard the offending happened when Hayward was tucking the girl into bed.
In a police interview and pre-recorded evidence, the child confirmed the accusations, saying "James unbuttoned his pants".
She also said Hayward told her to keep the interactions secret.
Justice Stephen Lemonis said Hayward's actions were a gross abuse of trust committed for his sexual gratification.
"There is also no indication in the material before me that you have an insight into the causes of this behaviour," the judge said.
His trial in August heard the girl's family was aware the former television producer had often read her bedtime stories and her mother said she had not initially been concerned.
An older sibling found the girl watching animated Family Guy pornography on her iPad and video-recorded her stating Hayward had shown her how to search for it and abused her, triggering an investigation.
Hayward emailed a "suicide note" to his wife soon after learning about the allegations, which she passed on to police amid fears for his safety.
In it, he wrote: "In the coming days, I expect to be accused of child sex assault in relation to (a child), which I have done".
He also alluded to the warning his wife had given him before the allegations surfaced about getting into bed with the girl over concerns he could be accused of abuse.
During a police interview, Hayward denied the note was a confession, saying he intended to take his life to spare his and the victim's family from media scrutiny that would follow the situation being made public.
The former pastor also said he didn't want his wife to blame his victim for his death, and the admission was designed to avert this and maintain the two families' close relationship.
Hayward also said he was under a lot of pressure and depressed over a failed former business, which owed $440,000 to the tax office, when he wrote the note.
He was acquitted of two other charges of indecently dealing with a child under 13, including showing her how to search for pornography on the internet.
Hayward resigned from the WA Nationals after the charges were laid in 2021 but continued to sit in the state's upper house as an independent MP.
He was formally disqualified from WA's parliament after his conviction and his seat vacated.
Hayward will be eligible for parole after serving 16 and a half months.
1800 RESPECT (1800 737 732)
National Sexual Abuse and Redress Support Service 1800 211 028
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