Harrison’s 14 years in media relations for the royal family followed seven years in Whitehall comms and involved him in high-level issues.
These included the St Andrews Agreement, with which Charles secured a media-free zone around Prince William during his university years, and dealing with the first in line to the throne’s reputation in the years following the death of his former wife Princess Diana and his public appearances with Camilla Parker-Bowles, who became the Duchess of Cornwall after marrying Charles in 2005.
The EMEA chief executive of Weber Shandwick, Colin Byrne, said Harrison’s "strategic counsel, wide experience on high-profile stories and events, and modernising approach to communications will be of great value to our clients".
Weber Shandwick represents brands such as MasterCard, Mercedes-Benz, Barclays and Save the Children.
Harrison will have a full-time role at the Interpublic agency, reporting to Byrne, but his arrival will not change other reporting lines into Byrne.
"Finding new and engaging ways to communicate in order to protect and strengthen reputation has been at the core of my work for the royal family over the past decade," Harrison said.
"I'm thrilled to be able to apply my skills and experience in reputation management and media relations to exciting opportunities with Weber’s clients."
Harrison’s Whitehall experience included being assistant director of comms for the Department of Environment, Transport and the Regions during the first three years of Tony Blair’s Labour government, following three years as a press officer for the Home Office.
The departure of Harrison from the royal family’s operation follows soon after that of Ed Perkins, the press secretary to Princes William and Harry and the Duchess of Cambridge, for a senior adviser role at Weber Shandwick’s rival Portland.