George Bennett established MDM in 2006 and moved the business to the LSE two years later.
He has spent more than 25 years in mining and finance, was the former CEO of Tanzania gold miner Shanta Gold, and also led OreCorp and Argentum Energy. He has also been a partner and director with Fergusson Bros, Simpson Mckie, and HSBC Securities Africa.
"We are delighted to bring someone of George's calibre on board," Rainbow chairman Adonis Pouroulis said.
"George has considerable experience in developing assets, particularly in the natural resources and energy space, and has a track record of success. We think he will be instrumental in helping Rainbow reach the next stage of its development. At the same time, I would like to thank Martin for his support during the early stages of Rainbow's development, IPO, and initial production."
Bennett said Gakara was "a truly unique asset, with enormous potential, which is as yet largely untapped".
"My focus will be on setting out a clear strategy based on two areas: stabilising production at Gakara, and developing an understanding of the deposit through a programme of exploration, combined with a robust study of how best to develop this exciting project."
Rainbow's share price has dipped about 25% this year to its current 3p, though it did get to 7.6p in June. It was at 12p a year ago. The company's current market value is about £11.4 million.