Representing Dripping Springs and Driftwood as well as portions of Kyle and San Marcos, Whisenant has served on the court since 2010 and said he would be 68 at the end of his second term.
The Republican unseated incumbent democrat Karen Ford in 2010 and ran unopposed in 2014.
Walt Smith, a Dripping Springs Republican from a ranch family, will run for the retiring commissioner's seat.
According to his website, his past experience includes working for the House Committee on Agriculture, working for the Department of Agriculture during the George W. Bush administration, running a consulting practice for nonprofits called the Mallard Group and working with the National Association of Agriculture, the National Association for the Advancement of Animal Sciences and the National Rural Water Association.
A Texas A&M University graduate, Smith also helped pass Proposition 6 in 2015, which amended the Texas constitution to protect the right to hunt and fish in Teas as well as the private property rights of landowners.
He served as a convention delegate for the Hays County Republican Party and is active in the Dripping Springs community.
Smith's campaign promises include lower property taxes, safer road and highways, public safety, conservative budgets and strong local businesses.
The Hays County Commissioners Court is set to see several shakeups in 2018, as former Precinct 3 Comm. Will Conley, who also chairs the Capital Area Metropolitan Planning Organization, has announced his run for county judge.
Bert Cobb, who currently holds the seat, said he would not run for re-election.
Conley's seat is also up for re-election, and two Republicans—Lon Shell and Colin McFarrin, have said they plan to file. Shell, who was the county's chief of staff, was appointed to replace Conley temporarily in October.
Filing for the March 6 Primaries begins Saturday and runs until Dec. 11, the earliest filing deadline in the country.