Lord Ribblesdale was Thomas Lister (1854-1925), 4th Baron Ribblesdale. He was Master of the Buckhounds from 1892 to 1895, Liberal Whip of the House of Lords, Lord-in-Waiting, and a Trustee of the National Gallery, London, from 1909 until his death.
Sargent first met him at an Artists’ Benevolent Fund dinner then stayed with him at his house in the summer of 1899. This was an un-commissioned portrait and Sargent was trying to capture the archetype aristocratic and he fully succeeds.
The painting was done in Sargent's Tite Street studio and you can see the parquet floor and how Sargent had him stand up against the classical molding of his wall which adds to regal feel -- a man fully in control of himself and his environment.