He joined the Ministry of Defence last week, to work alongside Reid's other special adviser, Steve Bates.
Arnold-Forster formerly advised Labour on defence policy, when the party was in opposition, from 1988 to 1994. He replaces Paul Corrigan, who resigned to become a public sector consultant after the general election.
The MoD special advisers to Reid's predecessor Geoff Hoon - James Connal and Michael Dugher - are advising him in his new position as Leader of the House of Commons.
PI is on the hunt for a replacement for Arnold-Forster, whose clients included VT Group (formerly Vosper Thornycroft) and the European Aeronautic Defence and Space Company.