Jason Megson, managing director, GPJ, said the agency wanted "people with entrepreneurial flair as well as experience with consumer-facing brands".
"This year will really be driven by our strategic and creative arms," he added. "Those two hires will help to transform the business this year and will be at the forefront of bringing in new clients in the UK and a more balanced portfolio of work."
Mason was previously creative director at Jack Morton Worldwide, while McCallum was head of sport marketing at Ogilvy & Mather Group UK.
See how GPJ worked with Mobile World Congress organisers GSMA to create its Innovation City in Barcelona last month .
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