Underhill made the announcement in a press release Friday.
"My supporters sent me here on a message of reform, and their continued support is the best indication that I have lived up to that expectation and have not succumbed to the constant pressure to do things 'the way they've always been done,' "Underhill said. "That is precisely why I will not be seeking a third term."
Underhill is in the middle of his second term on the commission after first being elected in 2014 as a Republican.
Only two commission seats are up for election in 2022: Underhill's seat and the District 4 seat currently held by Commissioner Robert Bender. Bender announced last week that he would seek reelection for his second term in 2022 as a Republican.