O’Donnell will be responsible for advertising across all print and digital elements of the Radio Times brand and will report to Immediate Media advertising managing director Matt Teeman, when she takes up the role on 15 May.
O’Donnell said: "Radio Times is a powerful brand with a fantastic audience that will only get stronger as the multimedia proposition develops."
Mercer departed in December following the Christmas double issue, just weeks after the completion of the transfer of Radio Times and several other magazines to private equity company Exponent, which named the new magazine company Immediate Media.
Speaking of his departure, Mercer said: "Earlier this year, after 12 years at the helm in the Radio Times ad sales team, I took the decision that the time was right to move on – and the change of ownership at BBC Magazines seemed the opportune time to do so."
Radio Times' ABC for the July-December 2011 period was 960,839 copies, down 3.7% year on year.
Follow Matthew Chapman at @mattchapmanUK