Initially the operation will publish Rodale's two titles but further launches in the health, well-being and fitness market have not been ruled out.
The company, owned 50:50 by NatMags and Rodale, will be run under a joint board of directors, with the appointment of a general manager to be announced imminently. No changes to editorial or advertising teams are expected.
Duncan Edwards, the managing director of NatMags, said: "Rodale's magazines have a better-than-average growth potential and the muscle of NatMags will help this.
"Together, our companies have identified long-term growth in the UK health and fitness market, and between our in-depth market knowledge and Rodale's 60-year history in the category, we will be perfectly positioned to respond to growing consumer demand in this area."
Managing the integration of the two companies will take first priority, with the move of Rodale's operations into NatMags' offices scheduled to happen in the summer.
Men's Health is a well-performing title in the men's monthly section, currently in fourth place behind FHM, Loaded and Maxim. In this year's ABC figures, it outperformed all three of these titles, recording a 0.8 per cent circulation increase.
Men's Health will sit next to NatMags' upmarket men's title, Esquire, in the men's monthly sector.
Runner's World will be a unique addition to its portfolio.
Edwards added: "Men's Health and Runner's World and the NatMags portfolio will complement each other perfectly. We will maximise cross-promotion opportunities, including marketing, subscriptions and advertising sales."