The Belfast-headquartered company has begun work on site after being appointed as the main contractor on the £35.5 million project.
The Gallery’s biggest-ever development since the building opened in 1896 at St Martin’s Place London, will include extensive refurbishment of the existing Grade I listed galleries.
The project, due to complete in 2023, will also include a new entrance and forecourt, which will transform the original building.
Gilbert-Ash Managing Director, Ray Hutchinson said, “We look forward to working closely together, and indeed imaginatively, with the National Portrait Gallery, the architects and the other disciplines and our trusted supply chain to deliver this prestigious project.
“Our team has the knowledge and experience to make a lasting difference to this iconic building for future generations and we are really excited to start works this month.”