"This is Sargent's largest and most impressive Full-Length Portrait in a Landscape Setting"  (The Huntington's description on notecards and posters)

From: Traditional Fine Art Online 

"Pauline Astor (1880-1970), who sat for her portrait by Sargent, was the eldest daughter of the Amencan financier William Waldorf Astor, who emigrated from New York to London in 1891. Miss Astor is shown dressed in a virginal white gown and silver-blue, fur-lined cioak, holding a fur muff; she stands in an autumnal garden and is accompanied by her King Charles spaniel Mossie, who tugs at her cloak. Although only in her late teens and not yet married, this self-possessed, elegant young woman had become mistress of her father's estate, Cliveden, upon the death of her mother in 1894. Despite her adult responsibilities, Miss Astor is presented by Sargent as a young woman in all of her youthful beauty, with her future fully ahead of her." 

(Traditional Fine Art Online)

Pauline's brother, William (Jr.) would marry Nancy Witcher Langhorne whom Sargent painted in 1909

Notes: 

Sold Sothebys; 12/3/1997; Lot 13; $1,982,500