Mahon, who was CEO of Shine Group from December 2006 to July 2015, is the first woman to take charge of Channel 4 and the first female boss of a UK broadcaster other than Channel 5.
She replaces the outgoing David Abraham and formally take over in October or November.
Channel 4 chairman Charles Gurassa described Mahon as “an outstanding leader and a highly experienced CEO who has developed and grown major international businesses in both the creative and technology sectors.”
He added: “She brings an impressive combination of relevant experience and is brilliantly placed to help steer Channel 4 through the competitive challenges and opportunities ahead. She is a great enthusiast for Channel 4 and its unique public service remit and her recent work with government will be valuable in facilitating constructive dialogue with the new administration.”
Mahon added: “Channel 4’s unique remit to innovate and to appeal to young and diverse audiences make it an essential part of British culture. There is nowhere in the world like Channel 4 and, in these changing times, its mission is more important than ever.
“I’m incredibly proud to be joining Channel 4 and bring to it experience both of leading creative organizations at scale and dealing with an environment of constant technological change.”
Scottish-born Mahon faces a tough job at the broadcaster. She will immediately face negotiations with the government over the relocation of Channel 4 out of London and has to search for a new creative chief following the resignation of Jay Hunt.
Hunt walked when she realised the job was going to an external candidate. In 2015, Channel 4 opened a new hub in Manchester, where it bases its regional sales team.