Palmer said she would leave her post as soon as Grant Thornton had appointed a successor, adding that she expected the recruitment process would take as long as the rest of this year.
But Palmer will stay on for a further 18 months as a member of the board.
She will assist her replacement in familiarising themselves with the firm before they take a position on the board at a yet-to-be-determined date.
Palmer joined the company in 1988 as UK marketing director, a post she held for ten years before taking on her current role.
She was previously head of external relations at Charterhouse.
Palmer's role at Grant Thornton is a daunting one. Under the partnership structure Grant Thornton International acts as an umbrella group for the auditor's largely autonomous offices in 110 countries.
Although Palmer might influence marketing and comms heads, they are not obliged to follow her lead. Much of her role involves centralised branding and internal comms such as a worldwide intranet system.
Last year it emerged that Italian subsidiary Grant Thornton Spa had testified to the credibility of the accounts of a Parmalat subsidiary.
Grant Thornton Spa was subsequently expelled from Grant Thornton International.