King Diamond's first heavy rock band was called Brainstorm (1974–76), with Jeanette Blum (Jean Blue) vocal and bass, Michael Frohn (Mike West) guitar and Jes Jacobsen (Jesse James) drums. King Diamond left Brainstorm and began singing with local Danish hard rock band Black Rose. It was during this time in Black Rose that King Diamond began experimenting with horror-themed theatrics as well as shaping a malevolent quasi-Satanic stage persona. He left Black Rose and joined the punk-metal band Brats where he met Hank Shermann. Soon after, the two of them were asked to help Michael Denner (also formerly a member of Brats) with his own project Danger Zone. This band included Timi Hansen and the mentioned musicians would join with King Diamond in 1980 to become Mercyful Fate.
Following Mercyful Fate's 1984 release of Don't Break the Oath and the subsequent tour (which saw them play in the U.S. for the first time), King Diamond split ways with Mercyful Fate. With him, he took two of his bandmates (Hansen and Denner) to pursue a solo career under his own name.
Mercyful Fate reunited in 1992 (while King Diamond simultaneously continued his solo career) and recorded five more studio albums. In 2000, King decided to put Mercyful Fate on hold and continue on with his solo career. Though Mercyful Fate remains dormant, the band is still under contract with Metal Blade Records. Also in 1999, King Diamond and Hank Shermann performed the song "Evil" live with Metallica. This performance also featured King Diamond without his famous makeup. In recent interviews, King Diamond has stated that Mercyful Fate will record and tour again when the timing is right, saying "It's definitely not finished."
On 01 August 2019 Mercyful Fate announced a reunion, and an accompanying European tour.
In 2001, King Diamond worked out a deal with the band
Usurper to sing backup vocals on the song "Necronemesis" in exchange for them shifting their recording schedule around to accommodate the recording of Abigail II: The Revenge. In 2004, King Diamond contributed vocals to "Sweet Dreams", which was the final track on the album of Dave Grohl's heavy metal side project Probot. In late 2005, King Diamond appeared on the Roadrunner United – The All-Star Sessions album, contributing vocals for his song "In the Fire", which featured multiple Roadrunner Records musicians (past and present) working together to create individual songs. King Diamond also guested on the Cradle of Filth song "Devil Woman" in late 2005.
In April 2006, King reunited with Mikkey Dee (Motörhead drummer) at a sold-out gig at Kåren in Gothenburg, Sweden. In 2001 King referred to Dee as "one of the best drummers of all time and that's something that has bothered us since he left."
King Diamond released his album Give Me Your Soul... Please, on 26 June 2007. Following the release the band received a Grammy nomination in the "Best Metal Performance" category for the track "Never Ending Hill." He was forced to cancel a United States tour due to a herniated disk, causing severe back pain, which puts him in intense pain almost all of the time. He attributes the problem to the long stressful hours spent working on the album.
King made an appearance at Ozzfest on 9 August 2008 at Frisco, TX alongside Metallica, performing a medley of Mercyful Fate songs previously released on Metallica's Garage Inc. album. Earlier in the day, he also performed a cover of the Pantera song "A New Level" with Vinnie Paul, Scott Ian, Max Cavalera, and Nick Bowcott.
In 2009, King Diamond was revealed to be a playable character for the rhythm game Guitar Hero: Metallica, appearing with Mercyful Fate's song "Evil." Complete the song on any instrument and any difficulty to unlock him.
On 29 November 2010, King was taken to the hospital, where they discovered several blockages in his arteries due to his heavy smoking habit. They determined that he had several heart attacks and that he needed triple-bypass surgery. The surgery was performed successfully, and on 11 December 2010 it was announced that he was at home recovering. All his musical projects at that time were then placed on hold.
On 7 December 2011, King appeared on stage with Metallica at The Fillmore in San Francisco to celebrate Metallica's 30th anniversary.
In June 2012, King performed his comeback concert, on Sweden Rock Festival.
On Saturday 8 September 2012, King appeared along with Mark Tremonti on VH1 Classic's That Metal Show discussing his surgery and various details about upcoming events.
King then appeared on Volbeat's 2013 album, Outlaw Gentlemen & Shady Ladies, for guest vocals on the track "Room 24."
In August 2013 King Diamond performed at Open Air Bloodstock Festival UK.
Slayer was chosen to headline the 2015 Rockstar Energy Mayhem Festival. 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 & Kissing Candice also participated in the Rockstar Tour.
King 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.