Sim worked for Stephen Spielberg's Amblimation in London during the early nineties. In 1995 he moved to Los Angeles as one of the first employees of DreamWorks Animation, working on the Prince of Egypt and then cutting the Oscar winning Shrek and Shrek 2 for director Andrew Adamson, as well as helping out on Madagascar.

Sim continued working with Adamson at Disney in New Zealand, editing The Chronicles of Narnia; the Lion, The Witch, and The Wardrobe and returned to the UK in 2007 to cut the The Chronicles of Narnia; Prince Caspian. Sim went on to edit Hippy, Hippy, Shake for Beeban Kidron and Nanny McPhee and the Big Bang for Susanna White, both at Working Title. He spent some time in New Zealand, again with Andrew Adamson, cutting Cirque du Soleil's World’s Away 3D, shot in Las Vegas, and MrPip, starring Hugh Laurie, shot in Papua New Guinea. Sim was consultant editor on the 3D effects movie Pompeii, and the animated movies Free Birds and Book of Life for Dallas' RealFX.

In 2014 Sim joined the Aardman Animations team in Bristol, for Oscar nominated, Shaun the Sheep the Movie. He then worked with the amazing Nick Park, editing his 2018 release Early Man. After that came Shaun the Sheep; Farmageddon which was also Oscar nominated.

In 2020 Sim moved back to London to help on Locksmith’s Ron’s Gone Wrong and is currently heading up editing on their Richard Curtis written, Simon Otto directed That Christmas, due out on Netflix Christmas 2024.