The $6.2 million building was approved by the county commission in 2007 as part of law-enforcement project that also included renovating the sheriff's office and the district courthouse.
The office is at 355 Miller Ave. next to the San Juan County Public Health Office.
The county bought the building that houses the existing district attorneys office, at 710 East 20th St., more than 40 years ago, County Executive Officer Kim Carpenter said.
The current office is undersized and outdated, District Attorney Rick Tedrow said.
He said frequent leaks would interrupt interviews and make it hard to store documents.
"We would fear the clouds rolling in," Tedrow said. "Continuous leaking caused damage to equipment and files."
County officials said the new building will have a positive effect on more than just the public employees who work there. Witnesses and victims, law officers and people who are on pretrial diversion programs come and go from the district attorney's office daily.
"We see quite a bit of traffic when it comes to people coming in," Tedrow said.
The office has about 65 employees. There are currently 15 prosecutors and the office is looking to add two to three attorneys, Tedrow said.
The office is currently working in the old building but plans to move to the Miller Avenue address by mid January.
The new building was designed by NCA Architects and built by B & M Cilessen. It is 26,000 square feet. The building has two stories but it is designed so a third floor can be added.
Carpenter said the county will have the old building appraised before making a decision about what to do with it.
Because of its age, it may be in taxpayers' interest to demolish the building and sell the land, he said.