Michael Gallagher is an American guitarist, best known for his work with post-metal band Isis. He joined them in 1999 for their debut LP Celestial, having previously been a member of Cast Iron Hike. His arrival marked a crucial period in Isis's development as they evolved into one of the most influential bands in the post-metal genre.

Gallagher's guitar work with Isis spanned over a decade and helped define the band's massive, atmospheric sound. He plays a '78 Les Paul Custom, which he bought on eBay, through a Fryette Sig:x Amplifier. His technical approach contributed to the band's distinctive wall of sound, as while playing in Isis he normally tuned his guitar down to (B-F#-B-E-G#-B), like his bandmate Aaron Turner.

Beyond his work with Isis, Gallagher explores entirely different sonic territories through his solo project. He has an ambient project known as MGR (Mustard Gas & Roses); under this moniker he has released two albums and scored one film. MGR represents a dramatic departure from the crushing heaviness of Isis, instead focusing on atmospheric textures, drone elements, and meditative soundscapes. This ambient project showcases Gallagher's versatility as a composer, demonstrating his ability to create immersive sonic environments that emphasize space, mood, and subtle tonal shifts rather than the massive dynamic contrasts that characterized his post-metal work.

Gallagher's contribution to Isis was instrumental in their evolution from their early sludge metal roots to the sophisticated post-metal sound that influenced countless bands. His guitar work can be heard on all of Isis's major releases, from Celestial (2000) through their final album Wavering Radiant (2009), helping to establish the template for modern post-metal with his layered, effects-heavy approach that emphasized atmosphere and dynamics over traditional metal aggression.