Pack will join Mandate as an associate director, charged with boosting the agency's digital PR offering.
His departure was announced internally earlier today, PRWeek understands.
As head of innovations for the Lib Dems, Pack is well regarded accross the blogosphere.
He played a key role in creating the party's digital presence, as well as managing the Lib Dems' internet campaign in both 2001 and 2005 general elections.
During his ten years with the party, Pack can take credit for a series of internet firsts, including the first use of Google Video by a major UK political party; the first UK party leader on Facebook and YouTube; the first national party website to provide RSS feeds and the first party to have an MP (Lynne Featherstone) regularly using Twitter.