Quinn was also a big part of helping the Duchess of Cambridge with her “early years” project, which aimed to steer children away from addiction and crime.
She is set to leave her role shortly after Christmas as she “wants more time for other interests.” the Daily Mail reported.
Ms Quinn’s decision to leave so soon is considered unusual because private secretary appointments usually continue for several years.
“Catherine is a thoroughly lovely lady and has achieved a lot working for the duchess. But she simply wants to take a step back and focus again on her charity work,” a source told the Mail. “Working for the duchess was actually never a job she had ever thought of taking on as she was scaling down her commitments, but then it came up and she couldn’t resist.
“She has done an incredible job but just wants some more time for other interests and feels she has helped the duchess create a solid platform in order to move forward,” the source said. “Catherine has helped to created such a strong team around the duchess, sources say, that she feels it is a good time to move on. They are parting on the best of terms.”