Charles Arthur Russell, Baron Russell of Killowen, (b. 10 November 1832 - d. 10 August 1900) was appointed Lord Chief Justice of England and Wales in 1894 which (at the time) was the second-highest judge of the Courts of England and Wales, after the Lord Chancellor. (wikipedia.org)
His title progression was legal counsel, then Baron Russell, and finally Lord Chief Justice.
Sargent appears to have been commissioned to paint two portraits -- one standing and one sitting only a year before his death. After his death he was asked to make a copy by the "Lord Russell of Killowen Memorial Committee Fund" in 1900 -- of the one sitting -- this one above. The photo above is of the replica. This replica was then presented to the National Portrait Gallery in 1921.The original is said to still be with the family.