Smith, who joined Mother in September, returns to Saatchis for the second time in his career. He will report to Richard Huntington, the agency's strategy director, and oversee the 17-strong planning department.
Smith, a former Campaign Face to Watch, will join the agency in the autumn. On top of departmental responsibilities, he will work on a number of still unassigned accounts.
Before Mother, Smith was a planning director at Bartle Bogle Hegarty, where he worked on Vodafone, Sony Ericsson and KFC.
Before that, he worked at Saatchis as a senior planner on accounts including Ariel, T-Mobile and Burton's Foods. He started his career at Bates UK as a planner on Ocean Spray and Hoover.
Huntington said: "Martin's hire is central to our commitment to building a phenomenal planning department here. He is a great cultural fit with the agency and our clients."