McDonald has 10 years of experience in mobile and web, both in the UK and internationally, including a stint as client services director at mobile marketing specialist, Sponge.
McDonald, who will report into Ross Taylor, chief digital officer at TMW, will spearhead the agency's mobile offering, offering practical advice on techniques, innovations and legal requirements.
He will also head up mobile marketing across the group of companies owned by TMW parent Creston, which includes ad agency DLKW and sales promotion agency The Real Adventure.
Taylor said: "Too often, mobile has been thought of as the add-on channel, but it is now too important to be treated as an afterthought.
"Right from the start we must consider how it can be integrated into our overall communications planning for the brands we support.
"We can deliver far better experiences for consumers by using the additional opportunities offered by mobile to engage at different stages of the purchase process.
McDonald said: "Our aim is to provide clients with a clear understanding of how mobile can work for their brand and their customers and how to avoid common pitfalls. As we work from a media neutral perspective we can ensure that they use it appropriately and maximise its effectiveness."