After eighteen years in the private sector at Gibson, Dunn & Crutcher, Republican Governor Pete Wilson appointed Grimes as a Judge of the Los Angeles County Superior Court. She served on that court's Executive Committee, Research Attorneys Committee, Media Committee, Bench Bar Committee, and ADR Committee. During her time on the Superior Court, she served as the judge in various lawsuits involving actors, agents, and others in the entertainment industry. In 2008, Grimes dismissed a paparazzo's civil claim for punitive damages against actor Keanu Reeves after the paparazzo was injured when Reeves drove his car out of a parking space. Later that year, she presided over a court case where the jury awarded a tow truck driver $5,000 in damages after he was assaulted by star Jason Wahler. In 2009, she ordered actor Jon Voight and his manager's parents to pay US$100,000 in attorney's fees to a pair of New Zealandproducers after the trio filed an unsuccessful malicious prosecution lawsuit against the producers. The producers had previously sued Voight and his manager's parents for fraud and breach of contract. In 2010, Grimes dismissed a lawsuit against Hollywood agent Ari Emanuel and his William Morris Endeavor Agency after the William Morris Agency's former reality television chief John Ferriter launched a lawsuit over various contractual violations and defamation related to the merger of the Endeavor Agency and the William Morris Agency. Outside of Hollywood-related cases, one of Grimes's final actions as a trial court judge was to dismiss a woman's lawsuit against a medical product manufacturer after the woman claimed that a medical camera fell with "crushing force" upon her knees. The woman had claimed that the manufacturer had created a defective product, been negligent, and committed a breach of contract, but an appellate court later ruled that Grimes was correct in dismissing the suit since the woman had no evidence supporting her claims against the company.
In February 2010, Governor Arnold Schwarzenegger appointed Grimes as an Associate Justice of the California Second District Court of Appeal, Division Eight, to replace Justice Tricia A. Bigelow, who had been appointed the Court's Presiding Justice. Grimes was confirmed two months later. In the November 2010 elections, 72.5 percent of voters supported retaining Grimes as an Associate Justice of the Court of Appeal.