Fisher's decision to step down appears a volte-face after she had opted, just a few months ago, to remain beyond her three-year term. She leaves with no job to go to, and while she's keeping tight-lipped about where she'll end up, the question on everyone's lips is who will take on her role.
Hers are, indisputably, formidable boots to fill. Despite the recent storm of bad publicity surrounding the size and purpose of the Government's advertising budget, her tenure will be most remembered for a steady improvement in the performance of COI.
Fisher got the organisation through the gruelling four-yearly review in February, continued to insist on the highest creative standards and conducted a fierce but fair review of its creative and media roster agencies.
But Fisher's position was undermined recently by the decision to extend her remit to advising 10 Downing Street's director of strategy and communications, Alastair Campbell. The move, which was intended to increase Fisher's influence within the Government and encourage her to remain in the post, instead erupted into a political firestorm which might well have pushed her toward leaving. The Government was forced to play down Campbell's role following complaints that he would overly influence COI.
The recent Panorama documentary - which suggested that the Government spent more on advertising in the run-up to the election, picking and choosing the most appropriate issues to boost its image before voters - was also damaging to the Government and, by default, the role of COI.
Fisher denies either of these incidents affected her decision to leave, saying her working relationship with Campbell was sound and that she had only remained beyond her initial three-year term because she wanted to see through the four-yearly review.
Nevertheless, Fisher's decision appears to have caught the Cabinet Office off guard and it will make no comment on possible replacements. It is currently deliberating over who to put on its five-strong shortlist of contenders for the job of managing 400 staff, a turnover of £295 million and complex relationships with government departmental clients and a huge roster of agencies.
Fisher, along with her predecessor, Tony Douglas, who ran COI for a couple of years, is from a marketing and agency background. The Cabinet Office's recent preference for candidates from the private sector reflects changes in COI, which is today a far cry from the department born out of the wartime Ministry of Information in 1946.
However, the majority of those running it have reflected its civil service roots and the chances of a successor being promoted from within are considerable.
Names that crop up include Charles Skinner, the head of communications at the DTLR, the department which controversially broke away from COI and formed its own roster of creative and media agencies earlier this year. A previous applicant for the job, some think Skinner will have a tough time convincing the Cabinet that he is right for the role after so publicly rejecting COI.
The current IPA president, Bruce Haines, remains adamant that the Government's main communications channel should continue to be run by an expert in that field, rather than a civil servant. "Advertising is a fast and effective way of getting crucial messages through to the public, and it makes sense to hand the role to someone who knows that market well."
A compromise may lie in the promotion of Fisher's deputy, the former advertising executive Peter Buchanan. Buchanan, if a little low profile, has experience of working both within the confines of the Government and the agency side.
Douglas says maturity, rather than dynamism, will be a crucial attribute: "It's a high-profile job which can't be done by a hotshot. It's not the right environment for anyone without considerable stature and experience. The COI job is a grown-up job."
But others claim that dynamism and motivational skills - the stuff of Top 30 agencies - are the things COI should be looking for. "COI is very like an agency in structure, and requires someone who can lead and inspire, another agency executive working on the roster says. "Carol did this brilliantly, but it seems like she wasn't given the support she needed. That said, the salary of £100,000 is a fraction of what a top agency figure would earn and could be a hindrance to attracting top talent.
Fisher claims that it will be tough for someone with no commercial experience to do the job, as it has changed in culture so dramatically. "I've worked hard to champion COI, and because it's now the biggest spender in the UK it has developed an attitude which befits that status. The organisation needs a strategist, as well as someone who will continue to fight for best value and the brand as a whole, Fisher comments.