Friend joined the Interpublic agency in 2010, initially heading its consumer marketing practice before rising to MD in 2015 and then CEO.
She has not said what her next move will be.
Weber Shandwick's London business has operated with a CEO and an MD since May last year when Helen Bennett was promoted to the latter role.
The move follows global leadership changes at Weber last summer, when chief executive Andy Polansky was named chairman and CEO of the holding company's Constituency Management Group division. Gail Heimann replaced him as global CEO at Weber.
Weber Shandwick is listed second in the PRWeek UK Top 150 Consultancies table published last year, with estimated UK revenue of £60m in 2018 and 387 UK staff.
In an internal note seen by PRWeek, Friend said: "As I approach my tenth year at Weber Shandwick, I have reflected on the amazing progress we have achieved. In partnership with our clients, we’ve transformed our reputation and our work, which has been recognised in the UK and globally with Cannes Lions, Global and EMEA PRWeek and Sabres, and most recently being named UK Large Agency of the Year at the PRWeek Awards.
"I have been lucky enough to work with some extraordinary people throughout the decade and the success we have enjoyed is testament to the hard work, passion and sheer talent of the team.
"It’s been a privilege to lead the agency through a period of real change, from helping to strengthen our core practice offers to helping to build our data, analytics, planning, creative and production teams.
"Recently, I was honoured to be appointed chairman of the PRCA, and as president of the PR jury at eurobest and a Cannes Lions juror, I have seen first-hand just how exciting a time it is to be in our industry.
"Now is the right time for me to hand over the reins to my very capable colleagues who I know will continue the agency’s successful path, and allow me to pursue some new, exciting and interesting challenges.
"I wish all my colleagues at Weber Shandwick every success for the future, and leave with deep thanks for the incredible opportunity given to me."
This story was updated on Tuesday morning to include the comment from Friend