John Marshall is a former guitarist for the thrash metal band Metal Church (1986–1993, 1998–2001). He was the guitar tech for Metallica's lead guitarist Kirk Hammett. Marshall also became the fill-in rhythm guitarist for James Hetfield in 1986 (while still performing his duties as guitar tech) while Hetfield's wrist was broken due to a skateboarding accident, and again in 1992 when Hetfield was recovering from pyrotechnics burn injuries stemming from a concert with Guns N' Roses in Montreal during the Guns N' Roses/Metallica Stadium Tour. He also briefly played in progressive thrashers Blind Illusion (which also featured Les Claypool and Larry LaLonde of Primus).
Marshall also has Type 1 diabetes. He stands 6'7".