The order, which will go into effect at 11:59 p.m. on July 7, requires residents to wear a face covering when inside public places and outside when social distancing is not possible.
In the announcement, officials said the mandate is key to ensuring businesses can remain open and schools can reopen in the fall.
"Mayor Anderson recognizes that there are many varying opinions on this issue, but believes that asking Williamson Countians to wear a face covering in indoor places and in outdoor places where distancing is not possible, is a necessary safety measure in order that our local businesses may remain open and our schools will be able to open in the fall."
As of July 5, Williamson County had more than 1,300 cumulative coronavirus cases.
The order comes just days after Gov. Bill Lee signed July 3 Executive Order No. 54, which enables county mayors to issue mask requirements.
The order includes certain exceptions for if the person is under age 12 or if a person cannot safely wear a mask due to a "bona fide" medical or health-related reason. Masks are not required while a person is eating or drinking or in a house of worship, although masks are still strongly encouraged in that instance.
The order is slated to last until at least Aug. 3.
Violations are a Class A misdemeanor, according to the order.
Read the full order here.