It started around 10 p.m. Thursday in the two-story structure that was undergoing renovations.
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"We are strong enough and we will rebuild," Mason County Judge Jerry Bearden told Austin's KXAN-TV.
All that was left by Friday morning was the building's rock wall structure. County records and documents had been relocated last year and were safe, KXAN reported.
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Firefighters had responded to a house fire more than a mile away from the courthouse at around the same time, but there was no word if the two were related, Bearden said. On Friday, a suspect was taken into custody after authorities said they believe the courthouse fire was intentionally set.
The Mason County Courthouse opened in 1910 after a year of construction.
"What we have left are 110-year-old walls," Bearden told KXAN. "It's a terrible tragedy for people in the county. This is my 19th year being in office here, and I've been through floods and fires, but I've never had anything with a gut punch that I've had with this."
Mason, a town of approximately 2,300 people, is 110 miles west of Austin and 270 miles from Houston.