William Lamb Picknell was a United States painter of landscapes, coastal views, and figure genres, known for his rapid painting style. He was born in Hinesburg, Vermont and died in Marblehead, Massachusetts.

Born in Hinesburg, Vermont, William was the son of Ellen Maria Upham and the Reverend William Lamb Picknell, a Baptist minister, both of New England families. In July, 1868, the summer following his father's death, the Picknell family moved to Chelsea, Massachusetts.

He began his career by working in a Boston frame shop. In 1872, he travelled to Europe where he trained with Jean-Léon Gérôme in Rome (1874–75) and also received some informal training from Robert Wylie in Brittany.

Throughout the 1880s, he primarily lived Waltham, Massachusetts, but frequently travelled abroad; spending two winters in England and also visiting other parts of the US, including Florida and California. He was living at Moret-sur-Loing, on the edge of the Forest of Fontainebleau, for most of the 1890s, but often spent the winters in the south of France. He returned to Massachusetts in 1897, where he died.