The pair had formed an unlikely friendship when they performed Eminem's song Stan together at the 2001 Grammys, NME reported.
It was around that time Eminem had been accused of homophobia, prompting the Gay & Lesbian Alliance Against Defamation (GLAAD) to protest outside the venue.
"He was accused of being homophobic by so many people because of his lyrics, which I thought was nonsense," John said on The Graham Norton Show this week.
"I came out and supported the fact that he isn't and I appeared on the Grammys with him."
John went on to tell the British talk show that he "adores" Eminem and that they have been close friends "ever since".