In 2002, Simpson co-founded Progressive bluegrass band Cadillac Sky in Texas with Matt Menefee (banjo), Ross Holmes (violin), Mike Jump (guitar, vocals), and Clint Sturgeon/Andy Moritz (bass, vocals).[3] They recorded their debut album Blind Man Walking in 2006 for a January 2007 release on the Skaggs Family label. They followed up their debut with Gravity's Our Enemy in 2008 and Letters in the Deep in 2010, which was produced by Dan Auerbach and released through Dualtone Records. Simpson departed the group in 2010 shortly after embarking on a tour opening for Mumford & Sons.
Following his time in Cadillac Sky, Simpson began a solo musical venture in Nashville, called The Whistles & The Bells.[4] Simpson self-produced and self-released the project's eponymous debut album in 2014. It was recorded over two days[5] with the help of Matt Menefee and Byron House among others. On August 7, 2015, the album was reissued globally by New West Records.[6] The project's second album, Modern Plagues, was released on April 28, 2017, promoted with the lead single "Harry Potter".[7]