Dedman will report to Jon Goulding, a founding partner of Atomic, and has been tasked with helping the three-year-old agency win new business as it seeks further growth this year.
After nearly three years at Mr President as business director, Dedman was the account director at Adam & Eve London, from 2011 to 2013, during which time it was bought by Omnicom and merged with DDB London in 2012.
At Adam & Eve she led accounts for John Lewis (including the launch of the 2012 "snowman" campaign), Sony, as well as the digital business for BMW and ITV.
Previously she spent four years as an account director at Dare.
She said: "I’ve watched Atomic’s progress with interest since it launched a little over three years ago. Its development in that time has been hugely impressive, but I know the team has bigger ambitions for both creative output and business growth on the path to becoming a leading integrated creative agency.
"I believe I can help them take this next step and begin to compete with some of the big name agencies in the industry."
Dedman’s appointment follows Atomic’s hire of Howard Willmott, the ex-DDB creative director, as its head of creative innovation, in November.
In December Atomic also hired Louise Weston, the former head of programmatic at Maxus, to be the managing director of its programmatic division, Atomic Live.
Goulding said: "Polly is a vital ingredient in our next chapter as we start to build a senior management team with top notch creative credentials from the advertising and digital world."