Originally from Winchester, Kentucky, Brenna MacMillan began playing banjo at the age of nine. Brenna performed frequently in the Lexington area which led to opening for J.D. Crowe and Bobby Osborne. Attending Berea College, Brenna played with the Bluegrass Ensemble under Al White and was awarded the Red Foley Memorial Music Award in 2018. After graduating with a bachelor's degree in Chemistry, Brenna moved to Nashville and started Theo & Brenna Band with her brother Theo. Their full bluegrass band toured for several years, releasing two albums. 

As Theo & Brenna Band stepped back from touring in 2022, Brenna began working as a sideman in Nashville playing with various artists and bands locally and touring, including Brennen Leigh, Paul Kramer, Hillary Klug, Greg Garing, Kimberly Perry and more.