Brannan will lead the Channel 4 News website, which was relaunched last September offering more depth and analysis, along with all social media initiatives and mobile applications.
As the BBC's first editor of emerging platforms, Brannan helped launch the corporation's first news site for mobile devices. Previously, he was editor of Ceefax and worked forPA and the Daily Express.
Brannan takes over from Ed Fraser, who is acting head of home news, and will oversee the "imminent" launch of the new Channel 4 News apps for mobile and tablet devices.
His appointment follows the announcement that the BBC's Matt Frei has joined Channel 4 News as Washington correspondent. Cathy Newman is the new regular presenter alongside Snow and Guru Murthy, while Newsnight's Jackie Long has joined as social affairs editor.
"Channel 4 News' online content is shaped around the fantastic journalists here - and their trademark depth and analysis," said Brannan.
"I look forward to extending the impact of this journalism from website and social media activity through to mobile devices - and exploring how we can further amplify the unrivalled content produced by the news team."
Channel 4 News deputy editor Martin Fewell added: "Paul's experience has arrived at just the right time for us - we've expanded Channel 4 News' impact and our relationship with the audience beyond that hour at 7pm - and this is the next stage.
"He has incredible form for spotting what's round the corner for news consumption and I look forward to him bringing that keen eye to our output."
Vicky Taylor, Channel 4 commissioning editor for new media, news and current affairs, said that the broadcaster has "some major new projects in the pipeline which will bring Channel 4 News to an even wider digital audience".
She added: "Having Paul's experience will help us target these new areas with even more impact."