Clements, who will join the agency on 2 June, will work alongside its managing director, Ben Simmonds-Gooding, and its executive creative director, Mick Mahoney.
He replaces Mustoes' head of planning, Jon Howard, who moved to Quiet Storm as its strategy director earlier this year (Campaign, 20 January).
Clements will now oversee planning on accounts including Butlins, Lloyds Pharmacy and the publisher H Bauer.
At M&C Saatchi, Clements headed strategy on accounts including Privilege, Scottish Courage and Lucozade.
Most recently, he worked on Kronenbourg's 2004 "sculptor/composer" TV ad and last year's Foster's "shadow" TV campaign.
Before M&C Saatchi, Clements worked at Banks Hoggins O'Shea, Bartle Bogle Hegarty and Heresy, where he was the planning director. He began his ad career on BMP DDB's graduate recruitment programme.
Separately, Mustoes has promoted the account director Charlie Barraclough to the position of new-business director.
Simmonds-Gooding joined Mustoes last May from Wieden & Kennedy Amsterdam, where he ran its Coca-Cola and Vodafone accounts.