This is the Anne, 2nd Countess of Albemarle by Sir Joshua Reynolds. Painted around 1760 in oil on canvas, she's approaching 60 years old here. The Countess holds a shuttle and is engaged in 'knotting', a pastime that involves making knots in thread to be sewn as decoration onto other items. You'll notice her face is almost dead-white due to the paint fading with light exposure.
Reynolds painted Lady Albemarle 11 times between 1757 and 1759, but this portrait wasn't finished until 1760. It's likely that it was commissioned by her eldest son, George, although her second son Augustus Keppel is believed to have suggested the idea of a portrait.
The Countess herself led an interesting life - she managed to fend off robbers in 1750 and even escorted a princess to Denmark for marriage. Her husband, William-Anne Keppel, was known for his extravagance and died in Paris as the ambassador.
Anne, 2nd Countess of Albemarle hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.