He is also a founding member and director of the London Musicians’ Collective.

Cusack is best known as a member of the avant-garde musical quartet Alterations (1978-1986; with Steve Beresford, David Toop, and Terry Day), and the creator of albums based on field and wildlife recordings.

Throughout his career, Cusack has been involved in a wide range of projects. Several of his pieces have been reviewed in the Leonardo Music Journal, the annual music journal published by MIT Press, for which he also curated an album. He is currently research fellow on the Engineering and Physical Sciences Research Council's multidisciplinary 'Positive Soundscapes Project'. Cusack is particularly interested in environmental sound and acoustic ecology. He has examined the sound properties of areas such as Lake Baikal, Siberia, and the Azerbaijan oil fields, and is interested in how sounds change with human migration and changing technology.

In 1998, Cusack started the "Your Favorite London Sound" project, whose goal is to discover which noises in their city Londoners find appealing. This project was so popular that it was repeated in Chicago, Beijing, Prague and other cities. He is also involved in the "Sound & The City" art project using sounds from Beijing in October 2005. Another project, "Sounds From Dangerous Places", collects sounds from sites that have suffered major environmental damage: Chernobyl, the Azerbaijan oil fields, and areas near the controversial dams on the Tigris and Euphrates river systems in southeastern Turkey.

Cusack's performances are a central part of the book Haunted Weather: Music, Silence, and Memory (2004), written by his former collaborator and respected music critic and author, David Toop. In his book, Toop investigates the use of environmental sound and electronic instruments in experimental music.

Cusack and clarinetist Simon Mayo together formed the duo known as "A Touch of the Sun". His first "major" recording was part of Fred Frith's 1974 record, "Guitar Solos". He was one of the first people in England to play the bouzouki, which gained him the respect of London's musical avant-garde. As a musician, he has collaborated with artists such as Clive Bell, Nic Collins, Alterations, Chris Cutler, Max Eastley, Evan Parker, Hugh Davies, Annette Krebs, and Eastern Mediterranean singer Viv Corringham. A live performance with Nicolas Collins was released as A Host, of Golden Daffodils in 1999.

In 1972, Cusack co-founded the artist-owned record label Bead Records, which has released many previously unavailable pieces. As of 2007, the label had released more than 30 albums.