Walker-Jones is the second major comms departure from Rolls-Royce after Barney Wyld recently departed for a role at National Grid.
At Lilium, he will focus on helping the business grow on globally and is responsible for comms across all elements of the business.
Lilium is developing an on-demand air taxi service and is designing and manufacturing Lilium Jet, the first all-electric vertical take-off and landing jet of its kind.
The business was founded by Daniel Wiegand, Sebastian Born, Patrick Nathen and Matthias Meiner in 2015 and is based near Munich.
"While the concept of an air taxi may have been around for decades, Lilium is tantalisingly close to turning what was once science fiction into a reality," said Walker-Jones.
"I’m genuinely thrilled to be joining a company of world-class engineers, remarkable designers, and top managers, and I look forward to playing my part in telling the Lilium story.
"We have the chance to inspire and excite people of all ages and backgrounds across the globe and I can't wait to make the most of that opportunity."
Arnd Mueller, Lilium’s VP of marketing and communication, added: "We are excited to attract the best talent to turn our vision into reality. With his wealth of expertise in key strategic areas, Oliver will be instrumental in communicating Lilium’s vision to our global audience."
Prior to working at Rolls-Royce, Walker-Jones spent several years with LafargeHolcim as head of external affairs and chief of staff. Earlier in his career, he held positions at Walmart, Blackstone and the CBI.