William Napier was the son of Francis and Nina Napier. Francis, also known as Lord Napier, served as the British Minister to the United States from 1857-1859. During this time the Napier family lived in Washington DC and were close with the Seward family. Fanny Seward recounts taking dancing lessons with William and his brothers in her 1858-1859 diary, and also seeing them during social visits. 

William became the 11th Lord Napier after his father's death in 1898 and held that position until his death in 1913. He married twice and had three sons with his second wife.