Garrett co-founded US tech agency 463 in 2004. In a blog post on the company website, he said that joining Twitter represented a ‘once-in-a-lifetime' opportunity.
‘I won't lie,' said Garrett. ‘Leaving 463 will be one of the hardest things that I will ever do. There just is so much emotional investment in it and so many good times with truly great people.'
He joins Twitter after consulting for the hugely popular microblogging service in recent months, and will build a ‘creative, smart and aggressive team' at the four-year-old website.
'I'm also eager to dust off my frontline media relations chops after spending most of the past nine years focused on the shadowy, back-room strategy thing,' he added.
Twitter's comms team, according to Read Write Web, already includes three other staffers: former Yahoo executive Jenna Sampson, ex-CNN correspondent Alexa Lee, and Emily Pinkerton.