The star, who played Greg Brady on the hit 1970s show, claims Elizabeth Kennedy made threats against his life and her own in August (09) after losing a beauty contest.
In legal documents obtained by TMZ.com, Williams describes how Kennedy also accused him of having an affair with a co-star in November (09) before attempting to grab a knife and attack him.
The 55 year old managed to avoid injury by overpowering his lover, but admits in the court papers, "Elizabeth is mentally unstable and has made threats to kill herself in the past and to harm me when she becomes depressed.
"I know that she will definitely try to harm me once she is aware that our relationship is now over."
Williams goes on to state that Kennedy is at the centre of a fraud investigation after reportedly stealing his bank card and taking $29,000 (£18,125) from his bank account.
He claims, "I have just discovered that Elizabeth made $29,000 in withdrawals from my Wells Fargo Bank account using my pin number which she found while apparently going through the papers in my apartment. Wells Fargo has photos of her at various ATMs of which they have just informed me and they are pursuing this through their Fraud Dept."
A Los Angeles Superior Court judge signed off on the restraining order request on 10 December (09), ordering Kennedy to move out of their shared home and banning her from going within 100 yards of the actor.