The agency was awarded the business this week after a pitch against the company's roster agencies.
It has been given the brief of launching a range of new Mars products in 2004 using a variety of integrated work, including through-the-line advertising, promotions and field marketing.
Michael Miley, the Joshua board director, said: "Getting appointed to Masterfoods' roster was an objective we had set ourselves at the beginning of the year. We are now looking forward to creating campaigns that herald from our unique integrated approach to communications."
Masterfoods is expected to launch a major assault on the confectionery market next year with a range of variants on its flagship Mars brand.
The company has already unveiled plans to launch an indulgent treat aimed at women called Mars Delight. This follows the recent launch of Mars Midnight, a dark chocolate, limited-edition bar.
The main Mars brand was relaunched last year with an ad campaign positioning the product as "Pleasure you can't measure".
Earlier this month, Joshua was appointed to handle advertising for Emap Automotive's Motorcycle News title.