A former child TV star who became a Harvard-trained lawyer has been accused of mounting a bizarre letter-writing campaign to topple a venture fund owned by Silicon Valley billionaire Peter Thiel.

Crystal McKellar, who played Becky Slater on “The Wonder Years,” allegedly tried to undermine Thiel’s Mithril Capital Management by penning inflammatory, anonymous handwritten letters to its investors — despite exiting the firm with a $225,000-a-year consulting gig, according to a suit filed this week in Texas state court.

McKellar, who graduated from Harvard Law School in 2003, served as the only general counsel of Mithril — a venture capital firm co-founded by Thiel and Ajay Royan in 2012 — before leaving in February.

Although her exit was also eased by a courtesy title, advisory managing director, she immediately “began a concerted whisper campaign to undermine Mithril, in which she would make false, anonymous complaints to Mithril’s limited partners,” according to the VC firm’s suit.

One letter, according to the suit, accused Royan of “lying to investors and the public about how much he is charging us in management fees.” Mithril was charging appropriate fees and McKellar signed off on, and was an author of, the disclosure documents,” the suit claims.

Mithril confirmed in September that it reached out proactively to government authorities in response to rumors of an alleged federal inquiry in order to protect investors and portfolio companies. At the time, a Mithril spokesperson called the probe “a foiled plot by a self-serving ex-employee.”

“I can state that the allegations of wrongdoing [in the lawsuit] are unequivocally false, and it will be a simple matter to prove them false if it gets that far,” McKellar said in a statement, adding that she hadn’t yet received Mithril’s complaint. “I left Mithril earlier this year when it became clear to me that Mithril’s leadership was lying to its investors and that the promises it had made were not going to be kept.”

According to the suit, McKellar mailed letters “written to sow discord between Mithril and its business partners.” The letters weren’t signed, but their envelopes sometimes contained a return address with the initials of the purported sender.

These initials allegedly “caused more than one recipient to assume the letters originated from one of Mithril’s more significant limited partners,” the suit claims.

Only after subjecting the letters to forensic handwriting analysis did Mithril identify McKellar as their author, according to the suit.

The revelation initially surprised Mithril, given the praise McKellar had heaped on Royan just before she left — including her writing that “he was a ‘person of great personal integrity’ who is ‘brilliant at tech investing,’ ” the suit states.

However, within a month of her exit, the suit claims, McKellar joined another VC firm — despite allegedly being bound to Mithril by a non-compete agreement.

The suit says McKellar took roles at a second, then a third fund called Anathem Ventures, information it says was disclosed in July in an e-mail from McKellar.

“She began soliciting Mithril’s limited partners almost immediately,” telling some of them the firm was being investigated by US regulators, McKellar’s former employer claims.

The breach of contract suit seeks more than $1 million in relief.