The annex houses every court in the county as well as offices for the magistrates, circuit clerk and sheriff, among others.
The entire courthouse complex will now open and close a half hour earlier from 8 a.m. to 4 p.m.
While some offices moved to the annex, others filled space in the old courthouse building.
The Lewis County Commission held its first meeting back in the courthouse Tuesday morning.
The commission will be meeting in the former circuit courtroom.
Commission President Pat Boyle said it's better for all involved to be so close to the rest of the county offices.
"As you've seen, it's just more conducive to do business. If you have a question or you need an item, it's at your fingertips within a matter of minutes instead of having to run completely across town. So once again, Agnes and Tom and I are tickled pink to be back here in the courthouse," said Boyle.
Commissioners are still working out how they'll use their new space in the courthouse, including how they'll arrange the courtroom for future meetings.
Other offices will move into the space in the old courthouse vacated by the sheriff's department.