He is known for writing, directing, and acting in audio fiction The Orbiting Human Circus (of the Air), and for performing with the theatrical troupe of the same name.

Koster joined Jeff Mangum, Scott Spillane and Jeremy Barnes to record the second Neutral Milk Hotel album, In the Aeroplane over the Sea, on which he played banjo and singing saw.

After Neutral Milk Hotel, Koster began to concentrate on his own solo project, The Music Tapes, for which he contributed banjo, singing saw, chord organ, and vocals, among other instruments.

He appeared in the Major Organ and the Adding Machine film which was shown on the Holiday Surprise tour and was later released with an expanded edition of the 2001 album in 2009.

[4] This evolved into an annual caroling tradition, which transformed into the "Lullabies at Bedsides" house tour of 2010–2011 and 2011–2012 which in turn gave way to the long-planned Traveling Imaginary of 2012–2013.

Koster wrote in his statement, "Anyone who knows me now or knew me then understands that I always try with all my heart to approach people with empathy and love, including Nesey, and over the last few years I've been working hard to build a friendship and to listen to her and acknowledge the pain that she feels.

My efforts have been met with increasingly disturbing behavior on the part of both Nesey and (spouse Paul Gallons), including thousands of threatening and manipulative messages, which are meant to punish me for not agreeing to what they want.

"[12] As Julian Koster With Chocolate USA With Major Organ and the Adding Machine With The Music Tapes With Neutral Milk Hotel Other Appearances