The son of Jan Małachowski, the royal grand chancellor, Małachowski was named marshal (speaker) of the Sejm (Diet) in 1788. He was the prime force behind a constitution, adopted in 1791, that embodied such modern western European reforms as majority rule in parliament, separation of powers, and enfranchisement of the middle classes; this constitution was abrogated at the Second Partition of Poland in 1792. In 1807–09 Małachowski served as president of the senate (government) of the Duchy of Warsaw, promoted by Napoleon.