[5] King Diamond's first heavy rock band was called Brainstorm (1974–76), with Jeanette Blum (Jean Blue) on vocals and bass, Michael Frohn (Mike West) on guitar and Jes Jacobsen (Jesse James) on drums.

In 2004, Diamond contributed vocals to "Sweet Dreams", the final track on the album of Dave Grohl's heavy metal side project Probot.

He also performed a cover of the Pantera song "A New Level" with Vinnie Paul, Scott Ian, Max Cavalera, and Nick Bowcott.

In 2009, Diamond was revealed to be a playable character for the rhythm game Guitar Hero: Metallica, appearing with Mercyful Fate's song "Evil".

On Saturday 8 September 2012, Diamond appeared along with Mark Tremonti on VH1 Classic's That Metal Show discussing his surgery and various details about upcoming events.

He appeared on Volbeat's 2013 album, Outlaw Gentlemen & Shady Ladies, providing guest vocals on the track "Room 24".

The bands HELLYEAH, King Diamond, The Devil Wears Prada, Thy Art Is Murder, Jungle Rot, Sister Sin, Sworn In, Shattered Sun, Feed Her To The Sharks, Code Orange and Kissing Candice also participated in the Rockstar Tour.

Diamond was a special celebrity guest star in four episodes of the Warner Bros. adult-oriented cartoon Metalocalypse as the Blues Devil, Ronald von Moldenberg, a fast food manager, and one of the Klokateers in 2006 on Adult Swim.

With Conspiracy, he wore a mesh of black and white line war paint, with some red "blood" made to look like a wound coming out of his forehead.

Diamond cites Arthur Brown, David Byron, Alice Cooper, Ronnie James Dio, Ian Gillan, Ozzy Osbourne and Robert Plant as his primary influences.

[17] According to Diamond's biography on his official website, the first two albums he bought were Deep Purple's Fireball and Black Sabbath's Master of Reality.

[21] Diamond is married to Livia Zita, a Hungarian-born singer who has made appearances as a backup vocalist on the albums The Puppet Master and Give Me Your Soul...Please, as well as during live performances.

She also helped him make remastered editions of the King Diamond albums The Spider's Lullabye, The Graveyard, Voodoo and House of God.

[30] American heavy metal band Cage devoted to him the song "King Diamond" on the album Hell Destroyer in 2007.