Maine Medicaid advocates sued Mr. LePage's administration in April after it missed a deadline to submit an expansion plan to the federal government.
The court said the Republican governor's administration has until June 11 to submit Medicaid expansion paperwork to the federal government.
The voter-passed initiative requires the state to start enrolling low-income adults into the expanded Medicaid program starting July 2.
"The governor can not ignore the law", said Robyn Merrill, executive director of Maine Equal Justice Partners, the lead plaintiff in the lawsuit and the group that led the ballot initiative campaign.
LePage spokeswoman Julie D. Rabinowitz, in an email response to a request for comment, said the administration is reviewing the court's decision.
"The Court is not persuaded that the executive branch is excused from clear statutory obligations by the legislature's failure to follow through with legislative obligations - as defined by the executive branch", wrote the judge, who was appointed by former Gov. John E. Baldacci in 2007 and re-appointed by Mr. LePage in 2015.
Yet fed-up voters demanded it in a ballot measure in November. But expansion supporters argued the governor's estimate was too high.
He said he wouldn't implement expansion unless state legislators found a way to pay for it without raising taxes.
This is why it's likelier that Utah will expand Medicaid by ballot box. "The Court concludes that the "complete failure to act" by [Commissioner of Maine Department Health and Human Services Ricker Hamilton] "cannot be considered substantial compliance" with the voter-approved statute". She also rejected the state agency's argument that it couldn't enter into a binding contract with the CMS to expand Medicaid without funding appropriated by the Legislature for the expansion.
ME voters approved expansion in November through a citizen-led ballot measure, but LePage has fought back at every turn.