Mahon was previously chief executive of Shine Group until 2015.
She replaces David Abraham, whose departure after seven years at the helm was announced earlier this year. At the time Abraham said he intended to stay until the end of 2017, but today Mahon confirmed she intended to take up the role in the autumn.
Jay Hunt, Channel 4’s chief creative officer, had been the frontrunner for the job until she announced her exit from the company three days ago.
Charles Gurassa, chair of Channel 4, said: "Alex is an outstanding leader and a highly experienced CEO who has developed and grown major international businesses in both the creative and technology sectors.
"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."
Since 2015 Mahon has been chief executive at Foundry, a global developer of software to deliver high-end visual effects, 3D, virtual and AR with clients including Disney, Sony Pictures, Facebook and Google.
She was previously chief executive of Shine Group, departing in 2015 after the entertainment company entered into a joint venture with Endemol to form Endemol Shine Group owned by 21st Century Fox and Apollo Global.
Having joined Shine as managing director in 2006, Mahon is credited with building the group through mergers and acquisitions as well as organic growth into a company that boasted 27 creative labels in 12 countries and £700m in revenue.
Unusually for a media executive, she began her career as a PhD as a physicist working in the NHS before joining strategy consultants Mitchell Madison Group as an internet retail expert. This began a career in media where she worked at Talkback Thames as chief operating officer, and director of commercial development and strategy at Frementle Media.
Mahon said: "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 organisations at scale and dealing with an environment of constant technological change."