She is set to join in June and takes the role vacated by Richard Woolfe, who announced he was to quit as director of television earlier this year to become director of programmes at BSkyB's Sky One, Two and Three.
Rosencrantz, who will report to Flextech managing director Lisa Opie, leaves ITV after 10 years at the commercial broadcaster, where she was responsible for a string of hits including 'I'm a Celebrity... Get Me Out of Here!', 'X-Factor', 'Who Wants to Be a Millionaire?', 'Pop Idol' and 'Ant and Dec's Saturday Night Takeaway'.
According to Opie, this track record as well as her experience in dealing with some of the biggest names in UK television, including Chris Tarrant, Simon Cowell and Ant and Dec, were key factors in her appointment.
Her role involves responsibility for programming strategy, commissioning, acquisitions, scheduling, marketing and PR, as well as overseeing the commercial and operations teams.
"She is one of the most talented commissioners in the market and will bring with her a wealth of experience and skill," Opie said.
Rosencrantz, who prior to working in TV was a print journalist working at the Sunday Telegraph Magazine among others, added that one of the main reasons for the move was the "challenge of multichannel... as we hurtle towards analogue switch-off".
Living TV hits in recent years have included 'Britain's Next Top Model', 'Extreme Makeover' and 'Most Haunted'. Its acquisitions include 'Grey's Anatomy', which premieres tonight on Five, its first outing on a UK terrestrial channel.
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