Chief District Judge John Dean swore in Marsh on Friday in front of an audience of local judges, lawyers and law enforcement officials.
Marsh was appointed to the Division 6 seat in the 11th Judicial District, which includes both San Juan and McKinley County.
Governor Susana Martinez formally appointed Marsh to the position Sept. 21. Marsh replaces former Judge Thomas Hynes, who resigned in June after a bar bustle.
"Was I expecting the appointment? I was hoping," said Marsh.
Marsh, formerly an assistant district attorney in San Juan County, has practice law since 1990. She has until November to preside over the court; she then will face Democratic opponent Emeterio Rudolfo, a private-practice lawyer in Farmington, for the position.
"It's very exciting for her. Nothing against her, but I do hope to win in November, so...," said Rudolfo, who attended Marsh's swear-in.
While Marsh is confident that she will be able to maintain her seat in November, her appointment could be undermined by a historically Democratic vote, according to Pat Cordell, chairman of the San Juan County Republican Party.
"We currently don't have any Republican district judges," said Cordell, who attributed the left-leaning line-up to past politics and the demographic of McKinley County.
He noted that there are, however, Republican party-registered magistrate judges.
Marsh considered politics a moot point in her new role, and instead deferred to her 10 years with the district attorney's office. "I just took the oath to be fair and impartial, so I think I can do just that," said Marsh.
During those 10 years at the district attorney's office, Marsh, an active member of the National Guard, spent three years deployed in Iraq. Marsh is also a Marine Corps veteran.
Whether it is cut short or not, Marsh is looking forward to her time as a judge, since it's a common aspiration for attorneys, she said.
"It's like going on an adventure. You don't know what's going to happen, so just take a step," she said.