By 1857, William was earning £35 a year as a butler, according to Wentworth Woodhouse Preservation Trust, and died in 1874.
Rose, 85, said: “My dad, Harry Denton Rose, inherited it from his mum. But my mum was scared by it – she said she felt William’s eyes followed her around the room and insisted it went in the attic.
“When it came to me, I had it cleaned and framed and hung on the landing.”
Although it was commonplace for wealthy families to have such paintings done of themselves, a portrait of a butler was more unusual.
Sheryl Hadfield, Denton’s great-great-great granddaughter, said: “Could he afford to pay for the painting?
“Or did an amateur artist paint him for free because he had an interesting face?
“We may never known, but we are so glad we have it. When I look into William’s face, I see a resemblance to my grandfather and my dad.”
The wider exhibition features old cinematic devices – from zoetropes to magic lanterns – with contemporary art installations.
It runs from Tuesday until 8 November.