Carr joins the transport lobbying specialist in August after eight years at BPPA and will report to Waterfront MD Nicolas Finney.
The hire is part of a post-election rejig at Waterfront and follows head of public policy Kerry McCarthy's election as the Labour MP for Bristol East. Deputy MD Mark Walker and head of political strategy Andrew Sharp have also left.
Journalist Anne Sack joined last week as senior client manager. She was most recently deputy syndication editor for The Times and The Sunday Times.
She reports to associate director Andrew Pakes, who joined a month ago.
He is an ex-adviser to deputy London mayor Nicky Gavron.