Jonathan Seward attended Marina High School in Huntington Beach, California. He proceeded through high school and formally met M. Shadows, Zacky Vengeance, Synyster Gates and Jimmy "The Rev" Sullivan. Despite not being in the band at the time, Seward already knew Gates and Sullivan, then soon after met Shadows and Zacky. Following the departure of Dameon Ash in 2002, he took the position as Avenged Sevenfold's bassist, as he had played the bass with the band whenever Dameon could not attend. Johnny could pick up on the pace of a song that he did not know quite quickly and proved to be a unique bass guitarist. "I don't think any one out there could pick up a bass line as quick as Johnny" were the words from the Rev. Johnny was also friends with Zacky's younger brother Matt Baker formerly from the band The Dear & Departed. 

Seward initially took a few weeks off from high school during his senior year to fill in for Avenged Sevenfold's previous bassist, Dameon Ash, during one of their tours because he went on tour with Avenged Sevenfold as a roadie. Those few weeks would become permanent as he dropped out of high school to become the band's current bassist.

He first recorded on their second album Waking the Fallen. He used to write only Johnny on his autographs until Zacky Vengeance came up with the name "Johnny Christ", and in All Excess he said, "That sounds... really ballsy. Everyone's gonna hate that, so... I should definitely go with that." He resides in Huntington Beach, Orange County.

In the time since the release of Nightmare, Seward has begun singing more backup vocals on live shows (as well as screaming) on songs like Nightmare, Seize the Day, God Hates Us, Not Ready to Die and Shepherd of Fire.