Department manager of sales, marketing, and corporate communications Donna Boland said the decision to hire an agency followed the expansion of the firm into a number of vehicle niches and price ranges.
Magnet won the work after a lengthy pitch process, which lasted nearly seven months.
Magnet senior vice-president and branding communications practice head Diane McCulloch said the agency plans to devote five PROs to the Mercedes team, with others helping as needed.
McCulloch will oversee the account, while Magnet CEO David Kratz will be involved on the brief on a weekly basis.
'What we needed was help in augmenting what we were doing internally, and expanding and leveraging those efforts,' said Boland.
Budgets are still being finalised, but Boland said fees will not exceed six figures.
Magnet will initially help refine Mercedes-Benz's media efforts, matching outreach for various models with publications that appeal to the target buying demographic, Boland said.
It will also work on gaining exposure for classic Mercedes models, playing on the firm's heritage as a carmaker.
The Magnet hire follows other moves to expand PR resources at the group.
Last year it created two new regional PR manager positions and hired industry veteran Geno Effler from Kia to handle PR for the US West Coast.
At the same time, it promoted Jim Resnick, a Mercedes veteran who had been handling product and tech PR, to head up communications in the East region.