Lucy Manning was a servant; she married a carpenter, Walter Slater.
She entered the Whistlers' service in April 1869. In 1871 the Whistler household consisted of his mother and three servants , two Welsh girls, Mary and Elizabeth Lewis, from Thanydloes, Montgomeryshire, and the 19 year old Lucy Slater from Mountnessing, Essex. These served the 35 year old painter (Whistler called her 'the famous Lucy', #09093) and his 65 year old mother.
UK census 1871.