Moroney, who took up her role this week, joins from Citizen, where she looked after strategy for clients including Cancer Research UK and Sky Movies.
At Mischief, Moroney will work with the agency’s two creative directors across its clients, which include Dulux and Unilever.
She said: "Mischief has a track record of creatively brilliant, commercially impactful work. I'm delighted to be here."
Moroney began her career at H+K in 1998 and later worked at Frank PR for seven years, where she worked with Mischief’s chief executive Frankie Cory.
Cory said: "Planning, like Mischief, is about messing with the norm to earn attention. Gemma joining the team takes our focus on creative work with commercial impact to the next level."