In late 2003, former lead vocalist of Underoath, Dallas Taylor (currently the vocalist for Maylene and the Sons of Disaster), left the band. Later in October of that same year at the CMJ Fest, Chamberlain took the stage as the new lead vocalist. During the early months of 2004, the band recorded their first album with Chamberlain and their second record for Solid State Records.

Chamberlain has released five albums with Underoath, They’re Only Chasing Safety (2004), Define the Great Line (2006), Lost in the Sound of Separation (2008), Ø (2010), and Erase Me (2018). When Define the Great Line was released on June 20, 2006, it sold 98,004 copies within its first week and debuted on the Billboard 200 Chart at No. 2. After They’re Only Chasing Safety, Chamberlain worked with Melissa Cross to add deeper screaming into the mix. Chamberlain then noted that his vocals on Define The Great Line sound less like imitations of Dallas Taylor’s vocals, and more similar to the vocals that he used in his previous band, This Runs Through. His evolved vocals came with the help of Melissa Cross, who coached Chamberlain and refined his “growl”. Alternative Press noted, “his bellow [is] more carnal and guttural, [and] his high end more tuneful than whiny. Now when Chamberlain’s relating those things that eat at the edges of his conscience and his soul, he has the kind of multi-faceted caterwaul to convey that despair.” The article also comments on his songwriting, saying that it “reaches inward, confronting every ugly crevice of his being, often emerging from the clash furious and disoriented, but hopeful.”

In 2010, Chamberlain made a guest appearance on the track “Quercus Alba” by To Speak of Wolves. The song is included on their debut album Myself < Letting Go. Chamberlain’s brother, Phil, is the drummer for the band.

In 2013, Underoath played their final tour before disbanding, though Chamberlain implied that he himself would pursue further musical endeavors. Later in the year, Chamberlain announced his new band Sleepwave, along with plans for a debut single, “Rock and Roll is Dead and So Am I”. In July 2014, it was announced that their debut album, Broken Compass, is to be released on September 16, 2014 via Epitaph Records which features their debut single, “Through the Looking Glass”.

In 2016, Underoath regrouped for a series of shows dubbed ‘Rebirth’ playing ‘They’re Only Chasing Safety’ and ‘Define the Great Line’ in full. They have since toured supporting Bring Me the Horizon.

However in a 2018 interview, Chamberlain stated, “I’m not saying religion is wrong for everyone, but for me it was wrong. It ruined my life, turned me into a drug addict and people were awful to me the whole time. I never felt more alone in my life than when I was Christian.”

On February 22, 2018, Underoath announced their eighth studio album, Erase Me, would be released on April 6, 2018; their first album in 8 years and their first in 10 years with founding drummer Aaron Gillespie. The first single was “On My Teeth.”