Grandson of a foremost civil judge, Hanworth succeeded to the viscountcy (and baronetcy) upon his father's death in 1996 and took his seat in the House of Lords until the House of Lords Act in 1999 removed his automatic right to sit in Parliament. He chose not to stand in the election by Labour hereditary peers to select two of their number to remain in Parliament after this Act came into force. Hanworth stood but was unsuccessful in the by-election caused by the death of Lord Milner in 2003. Willing to work in the Lords still, in 2011 he won the cross-house hereditary by-election to become one of fifteen 'deputy speakers', following the death of Lord Strabolgi, who was also Labour. He was therefore appointed/elected on the all hereditary-peer eligibility basis following a death of one of the 90 places which remain based on heredity.
He and his wife, Elizabeth daughter of Lawrence Vambe MBE, live in London. The title is expected to pass to a nephew as they have two daughters: