His role is being taken over by Peter Darbyshire, who joined PLB from Thierry's this summer.
Smith has been with PLB ever since chairman Jeffrey Fredericks set up the company in August 1982.
He said: "I have, in fact, worked with Jeffrey Fredericks for over 35 years, as we were at Luis Gordons together. When we started Private Liquor Brands we worked all hours, only got paid when there was money in the bank and dealt mainly in the own-label business - now over 60 per cent of our business is in brands.
"It's been an enormous privilege to watch the company grow and develop over such a long period of time, but now it's time to let the new blood take charge ."
PLB has adopted a new logo to mark the firm's 25th anniversary and is planing a number of special events to honour the landmark.