Monahan has created works ranging from piano compositions to loudspeaker installations since the late 1970s. He played in rock bands for several years, starting in the late 1960s. From the mid-1970s throughout the 1980s, Monahan studied physics, then piano and composition in Ottawa and New Brunswick, spending two years during this time as the pianist in John Cage's Études australes, and studying with Cage.
Monahan began mounting multimedia installations and constructing sound sculptures in the late 1970s. He often used natural forces in his work; the wind-activated string installation Long Aeolian Piano is an example of these kinetic sculptures. The 1980s also brought compositions for piano, including "Piano Mechanics." From the mid- to late 1990s, Monahan worked on "Multiple Machine Matrix" (MMM), a network of unique instruments made from recycled materials that are controlled via a MIDI computer.
By then, Monahan was based in Berlin, where he collaborated with Laura Kikauka not only on art projects but also on running an underground bar for a few years. Berlin also houses a permanent display by Monahan, his interactive work "Silicon Lagoon" (2000). Monahan has been commissioned by CBC Radio, the DAAD Festival, and more. One work, commissioned by an organization in Miami, was banned from the city's 1988 New Music America festival. "When It Rains" (2000) was commissioned by Canada Musique; "Earworks" (1983) and "Music From Nowhere" (1992) are among his other important works. Monahan plays organ in a pop group called Fuzzy Love, and is the artistic director of irreverent multimedia group KB Zed.