Leopold was a founder member of eclectic rock band Nojahoda who released one album on Sony S2 Records in 1999. 　　In 2003 he joined his brother Atticus Ross, as well as Bad Religion member Brett Gurewitz in the punk/hardcore band Error. Along with temporary vocalist Greg Puciato, the band recorded and released a well received EP on Epitaph Records in 2004. The group eventually disbanded due to a lack of full-time vocalist. Around this time Leopold also helped to compose the music for the USA Network show Touching Evil. 　　Leopold has since worked with artists including Bad Religion, Grace Jones, Korn, Dillinger Escape Plan, Rancid, The Transplants, Sage Francis, Brigitte Fontaine, and Brett Anderson working in various capacities including helping to produce and engineer records as well as being involved in songwriting and contributing some guitar parts. 　　In 2009 Leopold co-composed the score for the Hughes brothers' movie The Book of Eli. In May 2010 this score won a BMI film music award. 　　In 2009/2010 Leopold joined up with The Big Pink as a touring bassist on their world tour.