Magnus cut his musical teeth as a producer, songwriter, and arranger with a range of artists that defined the British fin de siécle musical zeitgeist from Pulp, Massive Attack, Morcheeba, Sinead O'Conner, and Neneh Cherry to chart-topping pop successes with All Saints (global number one Never Ever) and The Spice Girls, as well as helping to re-interpret the oeuvres of music icons such as Marianne Faithfull, Tom Jones, Gary Moore, Shakira, and  Bryan Ferry. Fiennes also produced  Damien Hirst's singular musical adventure, the chart-topping post-modern 1998 World Cup football anthem Vindaloo.

More esoteric recording outings include a 2002 collaboration with Canadian violin virtuoso Lara St John and India’s leading percussionist Trilok Gurtu on an album of genre-defying Bach arrangements for Sony, composing and producing three albums for all-female string quartet Bond on Universal and in 2016, the composition of a body of work for William Close and the Earth Harp, the world’s largest stringed instrument. 

As a creator of original content, Magnus conceived, produced and scored forty episodes of the groundbreaking music-driven BBC animation series Freefonix, nominated for best children’s animation at Annecy Film festival in 2008. In 2013 he teamed with immersive theatre company Punchdrunk to create music and sound design for its seminal National Theatre co-production The Drowned Man.

2011 marked the beginning of a collaboration with sister Martha Fiennes on SLOimage, an AI digital art platform which was launched with an inaugural work Nativity. In 2018 they completed the far more ambitious Yugen starring Salma Hayak which premiered at Venice Film Festival 2018 and has since been exhibited at Frieze, Christies NYC and LACMA, who acquired the work in 2019.

Magnus has lectured on film compostion and record production at London Film School, Kingston University and Morlely College London and presented a Tedx talk on inspiration. He is currently developing an inter-related slate of media projects that investigate the critical and precarious relationship between humanity and technology. These include conceptual art pop music act Deep Machines that will tour as a visually driven immersive, AR augmented experience, and a film drama about transgender electronic composer and synth pioneer Wendy Carlos.