Pattison has been with the South West England wholesaler since December 2008, when he joined as group business director after a long career with Somerfield. He was appointed managing director in April 2009.
Pattison oversaw the company's recent move into company-owned stores, including the opening of three large Eurospar outlets, two of which have subsequently closed.
He said: "I've very much enjoyed my time with the business, and thank the shareholders and staff for their support during my time."
Meanwhile, Appleby Westward has announced two new incentive schemes for its retailers. Under a training incentive scheme called WAR (Wessex Academy of Retail) every £2 spent at the wholesaler's depots will earn a point. Points can be redeemed against a range of training courses.
The second scheme, Maximise, will provide retailers with a 0.5% rebate for every £1 spent on chilled and frozen products from AW's Cullompton depot.