At the organisation's annual end of conference party - at which Tony Blair and Israeli Ambassador Zvi Shtauber addressed delegates - the body's director David Mencer introduced Purnell as the successor to Jim Murphy, MP for Eastwood, Scotland.
Purnell is a former Downing Street special adviser, but stood for parliament last year and won the seat of Stalybridge and Hyde.
It had been widely speculated that the role would go to former transport secretary Stephen Byers, but Mencer surprised LFI loyalists by announcing Purnell instead.
Mencer joined the organisation as director in July 1998. This followed a year-long stint as a consultant at transport PA specialist The Waterfront Partnership.
He took over the post from Miles Webber, who is now Merrill Lynch director of comms.