Henry Petty-Fitzmaurice, 3rd Marquess of Lansdowne (1780-1863), Son of the 1st Marquess by his second marriage, was born on 2 July 1780 and educated at Edinburgh University and at Trinity College, Cambridge. He entered the House of Commons in 1802 as member for the family borough of Caine and quickly showed his mettle as a politician. In February 1806, as Lord Henry Petty, he became Chancellor of the Exchequer in Grenville's "Ministry of All the Talents," being at this time member for the University of Cambridge; but he lost both his seat and his office in 1807. In 1809 he became Marquess of Lansdowne; and in the House of Lords and in society he continued to play an active part as one of the Whig leaders. His chief interest was perhaps in the question of Roman Catholic emancipation, a cause which he consistently championed, but he sympathized also with the advocates of the abolition of the slave-trade and with the cause of popular education. Lansdowne, who had succeeded his cousin, Francis Thomas Fitzmaurice, as 4th Earl of Kerry in 1818, took office with Canning in May 1827 and was Secretary of State for the Home Department from July of that year until January 1828; he was Lord President of the Council under Earl Grey and then under Lord Melbourne from November 1830 to August 1841, with the exception of the few months in 1835 when Sir Robert Peel was prime minister. He held the same office during the whole of Lord John Russell’s ministry (1846—1852), and, having declined to become prime minister, sat in the cabinets of Lord Aberdeen and of Lord Paimerston, but without office. In 1857 he refused the offer of a dukedom, and he died on 31 January 1863. Lansdowne’s social influence and political moderation made him one of the most powerful Whig statesmen of the time; he was frequently consulted by Queen Victoria on matters of moment, and his long official experience made his counsel invaluable to his party. He married Louisa, daughter of the 2nd Earl of Ilchester, and was succeeded by his son Henry, the 4th Marquess (1816—1866).

|- style="text-align: center;" | width="30%" |Preceded by:
William Sturges-Bourne | width="40%" style="text-align: center;" |Home Secretary
1827–1828 | width="30%" |Succeeded by:
Sir Robert Peel, Bt

|- style="text-align: center;" | width="30%" |Preceded by:
The Earl Bathurst | width="40%" style="text-align: center;" |Lord President of the Council
1830–1834 | width="30%" |Succeeded by:
The Earl of Rosslyn

|- style="text-align: center;" | width="30%" |Preceded by:
The Earl of Rosslyn | width="40%" style="text-align: center;" |Lord President of the Council
1835–1841 | width="30%" |Succeeded by:
The Lord Wharncliffe

|- style="text-align: center;" | width="30%" |Preceded by:
The Duke of Buccleuch and Queensberry | width="40%" style="text-align: center;" |Lord President of the Council
1846–1852 | width="30%" |Succeeded by:
The Earl of Lonsdale