The account manager takes charge of media relations for the firm's Scottish operations, including its oil and gas practice in Aberdeen and its financial services hub in Edinburgh.
She reports to UK head of media relations and comms Will White, who said her responsibilities would also include web comms.
MacDonald's first task will be promotion of E&Y's Entrepreneur of the Year Award in October.
She replaces Sarah Russell, who left in June for Fleishman-Hillard.
At Geronimo, MacDonald worked on campaigns for The Royal Bank of Scotland and National Lottery Good Causes, among others.