California Case Summaries

Koi Design v. Marron Lawyers — Professional-conduct breaches can support malpractice and fiduciary-duty claims

Reported / Citable

Case
In Re: Koi Design LLC v. Marron Lawyers, Apc
Court
Ninth Circuit Court of Appeals
Judge
Lucy H. Koh (Joseph R. Biden, 2021)
Date Decided
2026-08-06
Docket No.
23-55704
Status
Reported / Citable
Topics
legal malpractice, fiduciary duty, negligent supervision, professional conduct rules, causation, summary judgment

Background

Koi Design retained lawyer A. Douglas Mastroianni in a trademark case brought by Strategic Partners. While employed by Marron Lawyers, Mastroianni allegedly concealed major developments, missed deadlines, disobeyed court orders, and mishandled the defense. The trademark court entered default judgment and treble damages against Koi, contributing to its bankruptcy.

Koi sued Marron under California law for legal malpractice, breach of fiduciary duty, and negligent supervision. It claimed the firm failed to tell Koi about the litigation’s condition and inadequately supervised its associate. The Central District of California granted Marron summary judgment, concluding Koi could not establish breach or causation.

The Court’s Holding

In a published opinion, the Ninth Circuit reversed. California Rules of Professional Conduct do not automatically create civil liability, but violations may serve as evidence that a lawyer breached fiduciary duties or the professional standard of care. Marron owed Koi duties to disclose material facts and significant developments and to supervise its employees adequately.

Viewing the disputed record in Koi’s favor, a reasonable jury could find both duties breached. Evidence suggested the firm knew of Mastroianni’s serious problems yet failed to ensure the client learned the truth or received adequate protection. Summary judgment could not resolve competing inferences about what Marron knew and did.

A jury could also find causation. Koi had to show it probably would have achieved a better judgment or settlement with competent representation. The default and treble damages were tied partly to misconduct occurring during Mastroianni’s Marron employment, and Koi replaced him promptly once it discovered the mishandling. That evidence permitted an inference that timely disclosure or supervision would have led to competent substitute counsel and a better outcome. Judge Gilman dissented from much of the causation analysis but agreed the fiduciary-duty claim presented a triable causation issue.

Key Takeaways

  • Professional-conduct rules may be evidence of breach in California malpractice and fiduciary-duty litigation.
  • Law firms owe clients duties to communicate material developments and adequately supervise employed lawyers.
  • At summary judgment, disputed knowledge and competing factual inferences must be viewed for the nonmoving party.
  • Case-within-a-case causation may be shown through evidence that competent replacement counsel probably would have produced a better result.
  • A client’s prompt replacement of counsel after discovering misconduct can support the causal chain.

Why It Matters

The published ruling is important for California law firms and professional-liability counsel. Supervisory systems and escalation procedures are not merely internal management concerns; failures that leave a client unaware of serious litigation problems may support tort and fiduciary claims. Firms should document supervision, client communications, and remedial action when warning signs emerge.

For plaintiffs, the decision shows how to survive summary judgment without proving the ultimate malpractice case at that stage. Evidence tying firm-era misconduct to an adverse judgment, combined with proof that the client would have changed lawyers if informed, may create a jury question on the hypothetical better outcome.

Read the full opinion (PDF) · Court docket

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