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 his brother Raymond, Paul Barker, Roland Barker and Erich Werner. Eventually that band dissolved and Paul Barker joined the nascent Ministry. Rieflin's 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 ground-breaking album The Land of Rape and Honey. He was noted for his performance in the live video In Case You Didn't Feel Like Showing Up (Live) (alongside fellow drummer Martin Atkins). His work with Ministry and its side projects lasted through to the mid-Nineties, though he notes 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 labelmate Chris Connelly and founded First World Music. Like Connelly, Rieflin's work has grown 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 frequently overlooked keyboard skills.
Living in Seattle gave Rieflin the opportunity to build relationships with other prominent musicians including Robert Fripp and Trey Gunn of King Crimson, Scott McCaughey of the Young Fresh Fellows, Sascha Konietzko of KMFDM and Peter Buck of R.E.M. Fripp contributed to Rieflin's solo debut, Birth of a Giant, which also featured Rieflin singing in something other than a background role. Improvisations from these sessions turned up later on the CD The Repercussions of Angelic Behavior.
Rieflin appeared on all KMFDM records released from 1995–2003 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 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 plays synthesizers rather than drums. As the name suggests, its approach is geared toward the musicians leaving space for each other and doing a lot with few notes. All performances are improvised. The group played a small handful of live dates in 2005 and 2006 and went inactive for several years.
Rieflin is involved in a music collaboration project entitled The Humans, which consists of him, Chris Wong, Robert 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 on May 1, 2009. Another album came in 2013, followed by a small series of double-bill shows with Slow Music in spring of 2014.
Hector Zazou's 2010 album Corps Electriques featured Rieflin, as well as KatieJane Garside, Lone Kent and nu-jazz trumpeter Nils Petter Molvær.
Rieflin has been a regular contributor to Swans since the 1995 album The Great Annihilator, and has played an array of instruments on all their studio recordings since the band reformed in 2010 and released My Father Will Guide Me up a Rope to the Sky. (He is listed as an "honorary Swan" on The Seer).
In 2012, Rieflin performed drums for Robbie Williams's album Take The Crown. Later that year he produced the single Crush Vaccine for Atomic Bride. On December 15, 2012, Rieflin performed live with Atomic Bride for a sold-out show at Crocodile Cafe in downtown Seattle.
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 of 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.