Rieflin began his professional career in his hometown of Seattle. In 1975, he was in The Telepaths, a band which played backup for a couple of live gigs by the pre-The Screamers band The Tupperwares.He played drums for The Blackouts starting in 1979. His bandmates included Mike Davidson, Paul Barker, Roland Barker and Erich Werner. Eventually that band dissolved and Paul Barker joined the nascent Ministry. His earliest collaboration with Al Jourgensen was on the second single by the Revolting Cocks, You Often Forget.
Later, he participated in the creation of Ministry's album The Land of Rape and Honey, and was noted for his performance in the live video In Case You Didn't Feel Like Showing Up (alongside fellow drummer Martin Atkins). His work with Ministry and its side projects lasted through to the mid-nineties, though he noted that he was never credited as a member of Ministry proper, always as an "other" musician. Therefore, when he parted ways with the band during the Filth Pig sessions, he did not really quit since he was never an official member.
Rieflin helped Atkins kick off Pigface, the industrial collective that would grow to incorporate hundreds of artists, formed a friendship with labelmateChris Connelly and founded First World Music. Like Connelly, Rieflin's work grew beyond his industrial roots. They have collaborated on several recordings; two in particular, The Ultimate Seaside Companion (as "The Bells") and Largo, showcase Rieflin's keyboard skills.
Rieflin's solo debut, Birth of a Giant featured him singing in something other than a background role, and also featured Robert Fripp. Improvisations from these sessions turned up later on the CD The Repercussions of Angelic Behavior, which was credited to Rieflin, Fripp and Trey Gunn.
Rieflin appeared on all KMFDM records released from 1995 to 2003 as a drummer, programmer, vocalist and keyboardist. He toured with the band as a bassist in 2002 in support of its comeback album, Attak and performed on the 2011 KMFDM album, WTF?!. He also drummed for Scott McCaughey's band, The Minus 5, which occasionally included guitarist Peter Buck. Eventually Buck offered Rieflin the opportunity to sit in with R.E.M., who were missing a permanent drummer since the 1997 departure of Bill Berry. The band gave him the live drummer slot in its 2003 tour. They later announced that Rieflin would fill the role indefinitely, though once again as a hired musician rather than as an official member. In recordings, Rieflin also contributed bouzouki, keyboards and guitars to the group, serving as an auxiliary member until R.E.M. disbanded in 2011.
Rieflin formed an experimental ensemble under the name Slow Music in 2005 (including Fripp and Buck) in which he played synthesizers rather than drums. The group played a small handful of live dates in 2005 and 2006 and became inactive for several years. He was also involved in a music collaboration project entitled The Humans, which consisted of him, Chris Wong, Fripp and Toyah Willcox. The band performed a series of live dates in Estonia in Autumn 2007 and 2009, and released their debut album We are the Humans in 2009.Hector Zazou's 2010 album Corps Electriques featured Rieflin, as well as KatieJane Garside, Lone Kent and nu-jazz trumpeter Nils Petter Molvær.
Rieflin drumming behind R.E.M. on the band's final tour in 2008
In an online diary entry dated September 6, 2013, Robert Fripp announced a new lineup for King Crimson that included Rieflin as one of the band's three drummers. A few days after the first full-length live release of the band with Rieflin on board (Live in Toronto 2015), Fripp announced Rieflin's decision to take a sabbatical from the band, effective March 6, 2016, "a decision supported by all the Crimson Brothers." In early 2017, Fripp announced that Rieflin would be returning alongside his replacement, Jeremy Stacey. Due to Fripp's desire to stave off complacency after several years of touring, Rieflin "will be focusing on mellotron, keys and fairy dusting, rather than using drums as a main instrument" in the new Double Quartet configuration, thus becoming the band's first full-time keyboardist ever. For the US Autumn 2017 tour dates in October–November, he was replaced by the Seattle-based musician Chris Gibson. He rejoined in 2018 before taking another indefinite sabbatical in 2019.