Sullivan will report to H+K's chief creative officer Simon Shaw as part of an expansion of the firm's Global Center of Creative Strategy.
She will work across the agency's eight vertical sector specialisms and on projects to ensure its ideas are rooted in a data-led approach.
Prior to the appointment, Sullivan was the executive director of planning at Golin, where she led the creative planning team working across the company’s consumer, social, corporate and technology practices, as well as new business.
Clients included Unilever, nPower, Merck, ABInbev, Diageo and Mondelez/Cadbury.
Shaw said: "Eleanor will support creative strategy and planning for our clients, ensuring our data-led approach to creativity is evident in everything we do. I am delighted to welcome her to the Center of Creative Strategy based here in London."