
St. John was married and divorced twice. He had a son, Julian (1989–2014) and a daughter, Paris Nicole (born 1992) with his first wife, boxer Mia St. John. Julian died by suicide on November 23, 2014 following a long history of mental illness. He was a vegan and animal rights advocate and appeared in two PETA ad campaigns. St. John was married to Allana Nadal from 2001 to 2007, and they had a daughter, Lola (born April 15, 2003). On August 31, 2018, he was engaged to Russian model Kseniya Olegovna Mikhaleva. St. John died at his Los Angeles, California home on February 3, 2019. No cause of death was reported, but police suspected it was an alcohol overdose.

St. John's first major soap role was Adam Marshall on the NBC soap opera Generations. After the show's cancellation in 1991, he originated the role of Neil Winters on The Young and the Restless, and played the character for 25 years; no African American actor had appeared on the series more frequently than St. John. In 1992, he won the Daytime Emmy Award for Outstanding Younger Actor in a Drama Series for his role. On September 5, 1994, he hosted CBS Soap Break. In 2005, St. John became a special host for TV Guide Channel. In 2007, he received his fifth Daytime Emmy nomination. He was nominated for Outstanding Supporting Actor. In 2008, St. John won his second Daytime Emmy, as Outstanding Supporting Actor in a Drama Series.[citation needed]