Brennan, who will take up the position in September, will lead a team of strategists and researchers responsible for ensuring that the information and advice coming from the body is "authoritative, groundbreaking and enlightening". He will report directly to new Thinkbox chief executive Tess Alps.
Brennan has served at Associated News & Media as head of group strategic insight since October 2005 and leaves after less than a year in the role.
Brennan said: "I have only been at Associated a short time but have enjoyed it immensely.
"This is probably the only position within the media world for which I would have left such an exciting challenge."
Prior to joining ANM, Brennan set up his own independent media consultancy in Australia, providing media research, marketing and interactive content development advice to a range of major broadcaster and interactive media clients worldwide.
He has also worked on a number of projects for HTV, Yorkshire-Tyne Tees Television, ITV and Flextech.
Alps said: "David is one of those rare gurus who is both visionary and creative, yet can get surprisingly enthusiastic about the intricacies of media research. He has impeccable credentials, can see the big picture and absolutely adores TV."
Alps is due to take up her position as chief executive of Thinkbox next week.
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