She will take the post in February next year, replacing Zoe Osmond, who left in October to join Oystercatchers.
At Barnardo’s, Tickell oversaw marketing, retail and fundraising. She took the additional role of acting deputy chief executive last year.
Tickell previously worked at ad agencies for ten years.
Nabs has also appointed Charlie Parkin, a fundraising consultant, as the director of media relations.
Ian Pearman, the chief executive of Abbott Mead Vickers BBDO and chairman of Nabs, said: "The CEO of Nabs has to handle an incredibly broad brief and Diana brings huge experience and great strategic nous to the role. Along with Charlie continuing the great progress Nabs has made building relations with the media sector, we have a dream team in place with all the necessary skills needed to address the challenges ahead."