He joined Nightwish for their 2002 album Century Child, following the departure of previous bassist Sami Vänskä. He was a prominent guest musician in Delain, a project involving many members of the gothic/symphonic metal community. He is the bassist and lends his voice to a number of songs. He also participated in the recording of Invitation, by Altaria, providing backing vocals. Hietala has also been part of the bands Sinergy and Northern Kings.
Since his arrival to Nightwish, several songs have been written to contain duets with the former Nightwish vocalist Tarja Turunen, allowing songwriter and band leader Tuomas Holopainen to take advantage of Hietala's distinctive raucous voice to add a new dimension to the band. A famous example is Nightwish' cover of "The Phantom of the Opera", from the album "Century Child".
During Nightwish's shows, Turunen would take a break halfway through the set. Before Hietala joined the band, they used to perform an instrumental song during this time. Since Hietala joined the band however, they have performed covers of well-known songs, with Hietala singing the lead vocal part in this break. The band has performed Ozzy Osbourne's "Crazy Train", W.A.S.P.'s "Wild Child", Dio's "Don't Talk to Strangers", Megadeth's "Symphony of Destruction" and most recently, Pink Floyd's "High Hopes". Some of these songs have been put up for sale as well on various Nightwish album releases. Hietala has had much more involvement on the new Nightwish release, Dark Passion Play, which was released in September 2007. He sang some songs, completely, and wrote the song The Islander.
14th of December Hietala was chosen to the Artist of the Year in Musiikkilukio, where he did his A-levels.
Marco reportedly hates his plaited beard but feels that he cannot shave it because it has become so iconic (and his wife won't let him) .