Partners Andrews Aldridge and CCHM:Ping were ruled out at an earlier stage in a pitch conducted by Agency Insight.
Joshua will be charged initially with developing advertising and direct marketing to promote M&G's products in the UK and globally.
In addition to its consumer activity, the agency will be tasked with targeting M&G's growing intermediary business. M&G's spend is doubling from £1.44 million last year, according to Nielsen Media Research, to £3 million this year.
Jacqui Haskins, the marketing services director of M&G, said: "The brand has lots of ambition and we needed an agency that can adapt and work with us. Joshua is strong on brand and creative and has a maturity in its thinking."
M&G was previously handled by Harrison Troughton Wunderman, which declined to repitch. The agency was hired by M&G's former marketing services director, John Goddard, in 2001 and tasked with selling M&G's ISAs.
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