He was born Leandro Jan Ludwik Marconi on 23 April 1834 in Warsaw, then in Kingdom of Poland.
His father Enrico Marconi was a noted architect, who moved to Poland in 1822 and settled in Warsaw, while his mother was Małgorzata (English: Margaret) née Heiton, a lady of Scottish descent.
Marconi graduated from the local gymnasium in Warsaw and started career as an architect under the tutelage of his father.
(dubbed "palace under the artichoke"), the monumental seat of Bank Handlowy (1873), house of Stanisław Zamoyski, and a villa for Sobański family (1877).
The best-known of his designs - the Great Synagogue in Warsaw - was commissioned in 1877, after two successive architectural competitions were unsuccessful.