Noble's departure comes after nearly two years with the agency that she joined following a surprise exit from Cohn & Wolfe.
Weber Shandwick's CEO of UK and EMEA Colin Byrne sent a statement to senior staff that said Noble was 'leaving the firm to pursue other opportunities'.
He praised her 'valuable contribution' and her work on the Engaging, Always rebranding of the agency, which was rolled out this year.
His statement added: 'Given the new brand positioning is now complete, this is a natural time for both of us to come to this agreement. Fiona has been a great supporter of the London businesses and some key clients, and I have valued our partnership. We wish her well for the next phase of her career.'
Noble's move to Weber Shandwick filled the vice-chairman role left vacant since former Sun editor David Yelland joined Brunswick in 2006.
Her appointment came after she left C&W without a new job lined up, following talks with New York-based global CEO Donna Imperato.
Before C&W, where she spent four years, Noble was MD at Freud Communications.