Williams joined the University of Cambridge as a postdoctoral research associate working with Andrew Bruce Holmes and Richard Friend. Here she focused on the synthesis of electroactive polymers. She then moved to the University of Minnesota, working in the group of Marc Hillymer and William Tollman on zinc catalysis.
In 2003 Williams was appointed to Imperial College London as a lecturer. She was appointed a Senior Lecturer in 2007, a Reader in 2009 and a Professor in 2012. Here she developed sugar-based biodegradable polymers that were produced from lignocellulosic biomass. During her time at Imperial she was an inventor of several granted patents.
She joined Trinity College, Oxford, in 2016. Her research focuses on metal complexes for use in homogeneous polymerisation catalysis. She identified catalysts that could use of carbon dioxide as a raw material for polymers, which prompted Williams to start  Econic Technologies. Econic Technologies has received more than £13 million in funding. She also identified transition metal complex catalysts, biorenewable polymers and liquid fuel production. She has developed switchable catalysts that allow the combination of monomers into block copolymers. Working with Milo Shaffer at Imperial College London, Williams uses nanoparticles in polymer composites. She is a member of the London Centre for Nanotechnology.
She appears regularly in the media, including on BBC Radio 4s In Our Time' and at museums and festivals. In 2015 she won the WISE Campaign research award for her eco-plastics start-up.