Russell Seekins joins as the group communications director and will head the COI Communications and Tetley Tea accounts. He will report directly to Barry Cook, the agency's managing director and chairman.
Sarah Ryder has been appointed as a senior planning director. She will report to the executive planning director, John Poorta.
Ryder will work on Kellogg alongside Seamus O'Farrell. The former Soul partner is also joining Burnett as its group communications director.
The move follows the dual appointment of Mother's Jim Thornton and Paul Shearer from Wieden & Kennedy Amsterdam as Leo Burnett's executive creative directors.
Cook said: "Recruiting the best possible talent is a vital part of ensuring we deliver for our clients. I think Sarah, Russell and Seamus will deliver at the highest possible level."
Seekins joins from Ogilvy & Mather, where he was a managing partner sitting on the executive committee. He also headed O&M's COI team.
Ryder left The Leith Agency in 2002, having worked on the Carling and Iomega accounts. She has also completed stints at the Sydney-based Campaign Palace agency, M&C Saatchi and Butterfield Day Devito Hockney.