Penny Harris has been promoted from senior planner to planning director.
She will take over from Phil Teer, the joint managing director, to enable him to concentrate on running the agency.
Harris has focused on St Luke's Clarks business for the past three years.
Before that, she ran the brand strategy team at Interbrand.
Teer said: "Penny is a natural choice for the role. She has an astute planning brain and her experience on both the client and consultancy sides gives her a unique perspective on client problems."
Holly Day is joining the agency as its head of account management. She returns to St Luke's after five years following a stint in the US. Most recently she was the Nike account director at Wieden & Kennedy Portland. Before that, she worked at Goodby Silverstein & Partners.
The position of head of account management has been vacant for the past few months following the departure of Richard Howarth.
Neil Henderson, the joint managing director, has been caretaking the role.
Henderson said: "Holly is a veteran of some of the world's most creative agencies and her previous experience of St Luke's means she will hit the ground running.
"It's a great appointment for us and we are delighted to have her on board."