St. John died suddenly under murky circumstances in early April 2007, several months after being badly beaten during a brief stay in an Orange County jail.

Prior to joining Kiss in 1984, St. John worked as a teacher and played guitar in a Southern California cover band called Front Page.

He was selected as Kiss' new lead guitarist after an audition, and he subsequently signed a five-year contract with the band,[3] replacing Vinnie Vincent.

Gene Simmons had asked St. John to record a bass part and Carr later remarked to him that he thought one of the notes sounded wrong.

St. John's furious reaction left Simmons extremely unimpressed, and Carr "wanted to kill Mark" following the incident.

[3] Stanley said that, additionally, they had tremendous difficulty writing and recording guitar solos for Animalize, with St. John often being unable to play the same thing twice if a second take was required.

[4] Kiss issued a press release stating that St. John had developed severe reactive arthritis and was unable to play guitar.

In January 1985 St. John teamed up with vocalist David Donato and drummer Barry Brandt of Angel to work on developing some demo ideas.

Some of the musicians playing alongside St. John included the Steve Hooks Band, Stu Nevitt (Shadowfax), Slyde Hyde (Tom Scott/Supertramp), Al Aarons (Count Basie), plus other special all-star guests.

The band also included St. John's younger brother, Michael, on bass, but was completed with the addition of Brian James Fox on drums.

St. John also did some session work performing lead guitar on "Livin' for My Lord" on Ken Tamplin's 1990 album, Axe to Grind.

He was in a short-lived band with Phil Naro called the Mark St. John Project that released a limited edition EP in 1999, and he also made an appearance at a Kiss expo in New Jersey.

[8] Beginning on September 14, 2006, St. John was incarcerated for several days at Theo Lacy Jail in Orange County, California, after being charged with possession of unspecified drug paraphernalia, attempted destruction of evidence, and resisting arrest.

He was later moved to F-West Barracks after telling guards he had stolen crackers from another inmate's property box and was in fear for his safety.

A couple of days before the beating, St. John told her that he had "snitched" on a drug dealer several years earlier, and he believed he was certain to be attacked if this became known by the other inmates.