Loughran will become an associate in Freuds' corporate division, which is known as The Brewery and accounts for around a third of its 230-strong London staff.
He will work alongside director Hannah Pawlby on special projects, campaigning and events.
Loughran has been in the royal household working for Prince William and other royals for eight years, having previously been a corporate comms officer with the City of London Police.
Arlo Brady, MD of The Brewery, said: "Nick is an important hire for our growing Brewery team and brings valuable expertise in the areas of work The Brewery excels at."
In recent weeks, The Brewery has also welcomed new director Charlotte Budd from Burson-Marsteller and Hanover's Chris Calland, previously of PLMR, as an associate.
Most recently, Loughran worked under Jason Knauf, the duke and duchess' comms secretary, who took the job after leaving RBS in early 2015 and was named one of the 1,000 most influential Londoners by the London Evening Standard.