Williams will report to Natasha Murray, director of client partnerships, Guardian news and media, and will work closely with the Guardian's fashion editorial team.
He is tasked with growing the publisher's commercial revenues in the fashion category.
Williams will also be responsible for the Guardian's commercial relationships with key fashion houses and for working in partnership with key advertisers for The Guardian, The Observer and The Fashion, the bi-annual unisex style magazine.
"The Guardian’s coverage of fashion is at the cutting edge of contemporary culture and I'm excited to be joining a team that puts readers at the centre of everything it does," Williams said.
Murray credited Williams' appointment to his in-depth knowledge of the sector and strong international publishing experience.
Williams first joined the Financial Times in 2004 and has since held several positions including head of arts advertising and global client director for men's fashion.
At the FT, Williams relaunched its arts website in collaboration with advertisers. He most recently launched the Art of Fashion magazine as part of the refreshed FT weekend edition.