2ergo, which counts O2 as a customer, specialises in helping brands to maximise the potential of mobile marketing through developing smartphone applications, mobile publishing, interactive messaging and other products.
Along with O2, clients include Yahoo! and the National Geographic Channel, and it was behind Arsenal FC's recent iPhone application that topped the sports charts in the iPhone App Store.
MacDonald resigned from O2 in August. His position has since been filled by Shan Henderson.
Chris Brassington, group managing director at 2ergo, said: "Mobile is fast becoming a critical communications channel and the preeminent way of engaging and drawing audiences closer to the brand.
"2ergo has considerable experience of optimising and monetising media content for mobile and James's experience will be invaluable as we continue to build on our record of success in the media and entertainment market."
2ergo was started in Manchester but Macdonald will be based at 2ergo's London office.
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