Born Joshua Howard Luellen, he grew up, as his name suggests, in South Atlanta, focusing on sports before he started beatmaking at the age of 14, inspired by Lil Jon, Pharrell Williams, Dr. Dre, Timbaland, Kanye West, and Swizz Beatz. He forged an early alliance with Waka Flocka Flame, joining fellow producer Lex Luger on a trio of tracks from the rapper's 2010 debut, Flockaveli. That same year, Luellen and Luger joined to form the production/songwriting team 808 Mafia.
As Luellen's reputation grew in the Atlanta area, he started working with Gucci Mane, who would become another frequent client. With the Southside brand in demand, the rising producer went on to work with Kanye West, Jay-Z, Meek Mill, 2 Chainz, Rick Ross, Wale, Machine Gun Kelly, Pusha T, Young Thug, Drake, and many more. In 2018, Southside produced the entirety of Chicago MC G Herbo's sophomore effort, Swervo, which debuted at number 15 on the Billboard 200. In October 2020, the Blood on the Hills mixtape was released, showcasing new collaborative work from Southside and his 808 Mafia peers and Pyrex. Less than a month later, Southside teamed with Doe Boy for the sinister trap beats of the Demons R Us mixtape. More production work for other artists ensued, including work on Travis Scott and Future's 2022 track "Hold That Heat," which debuted at the number 57 slot on the Billboard charts. ~ Neil Z. Yeung, Rovi