Harris was serving a term that is not scheduled to end until March 2028.
The president is only allowed to remove leaders from the MSPB in cases of "inefficiency, neglect of duty, or malfeasance in office," and Harris' lawyer argued that Trump, who fired the board chair in a one-sentence email from the Presidential Personnel Office, did not make the case that Harris needed to be fired.
Contreras wrote in his ruling granting Harris a temporary restraining order that the MSPB "falls within the scope of Humphrey's Executor," a 1935 Supreme Court ruling that established the precedent that Congress can require the president to show cause before firing board members at independent agencies.
"Congress has the power to specify that members of the MSPB may serve for a term of years, with the president empowered to remove those members only for inefficiency, neglect of duty, or malfeasance in office," Contreras wrote.
"The president did not indicate that any of these reasons drove his decision to terminate Harris," he continued. "The court thus concludes that Harris has demonstrated that she is likely to show her termination as a member of the MSPB was unlawful."
A federal judge last week also blocked Trump from firing Hampton Dellinger, head of the Office of the Special Counsel (OSC), which protects whistleblowers.
If the cases make their way to the Supreme Court, the court's right-wing majority could rule in favor of expanding presidential powers related to dismissing the heads of watchdog agencies.
Since the MSPB hears appeals from federal workers, Harris' reinstatement is also "particularly noteworthy given the recent mass firings of [federal employees] across government," said Eric Katz of Government Executive.
After granting Harris a temporary restraining order, Contreras ordered Harris to submit a motion for a preliminary injunction within five days and scheduled a hearing on the matter for March 3.