Lord Arthur Marcus Cecil Hill of Ombersley, who was born at a house in Hanover Square, Mayfair on 10 January 1792, was the third son of Arthur Hill, 2nd
Marquess of Downshire (1753-1801) and his wife, Mary (1764-1836), daughter and coheir of the Hon. Martin Sandys by Mary, daughter and coheir of William Trumbull of Easthampstead Park, Berkshire. In 1802 Lady Downshire was created Baroness Sandys of Ombersley following the death of her husband, she having inherited the estates of her uncle, Edwin Sandys, 2nd
Baron Sandys who died without issue in 1797. When Lady Downshire died on 1 August 1836, the Sandys barony (of the second creation) passed to her second son, Arthur Moyses William Hill as 2nd
Baron Sandys (1792-1860), a bachelor. When he died the title passed to his next brother, Arthur Marcus Cecil Hill who succeeded as 3rd
In 1825, Lord Arthur was appointed secretary to a Special Embassy to Lisbon and then to Rio de Janeiro in order to broker a peace treaty between Portugal and Brazil, King John VI of Portugal's son, Pedro having declared himself Emperor of Brazil. That same year Lord Arthur was made a Knight Commander of the Portuguese Military Order of the Tower and Sword.