He will report to TBWA\London's executive creative director, Steve Henry, and will oversee the agency's art direction as well as work on a number of unconfirmed accounts.
Shillum joined W&K last year to work on The Guardian pitch, and was recruited permanently after the agency won the account in August.
He has held a number of positions at shops including Bartle Bogle Hegarty, where he worked exclusively on the Levi's European account for two years, and before that W&K Amsterdam, where he worked on the agency's Nike account.
Henry said: "Marc is one of the most exciting talents I have seen in this industry. His CV shows an unbroken list of blue-chip accounts. I'm looking forward to seeing how much change he can bring to an agency already changing on a daily basis."
Shillum added: "Matt and Steve have a great vision of how to build a true multi-disciplinary and integrated agency. On top of that, TBWA has an awesome advertising heritage."
Shillum takes up his new post on 19 February.