McDonald replaces Martin Bailie, who has been promoted to commercial strategy director.
Both will work across Isobar's four divisions in the UK: Glue Isobar, Isobar Mobile, Isobar Social and Superglue.
McDonald will lead Isobar's planning offering, working closely with Tim Dunn, the head of strategy at Isobar Mobile. Bailie will focus on driving new revenue streams for the Isobar UK business. Both will report to Penny Herriman, the chief executive.
At CHI, McDonald was a founding partner of its social media division, The Social Practice, working with clients including Argos, RSPCA and The Mobile Money Network.
Previously, she worked at Bartle Bogle Hegarty, where she was the business and planning director at BBH Labs as well as the planning director and global business director.