Ms Quinn first began working with the Royal mother-of-three back in October 2017, serving the Cambridge family for a little over two years.
Kate Middleton's personal secretary is involved in planing and scheduling the Duchess's appointments as we all accompanying the Royal on her official engagements, according to the Daily Mail.
Ms Quinn has also been credited with helping the Duchess of Cambridge promote her important work for 'early years', a program which aims to steer young people away from a life of addiction and crime.
Quinn is set to leave her position after Christmas, and will return to her former philanthropic pursuits as a part-time charity worker.
The news of Quinn's sudden departure from the Royal staff comes just days after Kate comforted a heartbroken mother at the East Anglia's Children's Hospices' (EACH) last week.
The mother-of-three met with families to hear about the care they receive at the hospice in Norfolk in the UK.
Mother Naomi Wright revealed that Kate tenderly hugged her when she broke down about her four-year-old son Rupert’s condition.
"EACH was one of the very first charities that I decided to become patron of after my marriage," Kate said in a moving speech.
"Whilst a lot has changed since then, my commitment and support for this wonderful organisation and the work that you do has not."
Kate has been a patron of EACH since 2012 and officially opened its hospice in Ipswich called The Treehouse that very year.