Allen is currently head of Robert Horne's Sign and Display division. He will replace Marchant, who was recently named as PaperlinX's regional president for the UK and Ireland.
Marchant is to take over from Martin Fothergill, who is relocating to Melbourne to become group general manager for merchanting Australia and New Zealand.
"His energy in driving through the radical re-organisation of Sign and Display so quickly has demonstrated that he is exactly the right man to lead Robert Horne forward," said Marchant.
Allen said he thought there were real opportunities for the group, and he was looking forward to the future with optimism and determination.
"For our customers and suppliers, I promise to maintain the excellent levels of service and commitment of this great business," he said.
As part of his new position, Allen will continue to take responsibility for the Sign and Display division until he has had time to appoint a successor.
In the meantime, he will report to Marchant, who will work closely with him to ensure a smooth handover.
Allen was appointed as a director of Robert Horne Sign and Display in 2004. Prior to joining the company, which is part of the PaperlinX group, he held a number of posts as managing director in the Saint-Gobain industrial conglomerate.
Hornes Allen replaces promoted Marchant
The Robert Horne Group has appointed David Allen (pictured) as its new managing director. He will take over the position from Toby Marchant from 1 November.