Legionowo is located 23 kilometres (14 miles) to the north-east of the center of Warsaw and only 7 kilometres (4 miles) to the south of Zegrze Reservoir (Jezioro Zegrzyńskie or Zalew Zegrzyński), near the Warsaw-Gdańsk railroad and Warsaw-Suwałki road.
Situated in the Masovian Voivodeship (województwo mazowieckie, created in 1999 as a result of Local Government Reorganization Act), previously in Warszawa Voivodeship (województwo warszawskie or województwo stołeczne warszawskie, 1975–1998) and old Masovian Voivodeship (before 1975). Currently this is the capital of Legionowo County (powiat legionowski), which is one of 38 land counties (powiat ziemski) in Masovian Voivodeship.
Legionowo's history dates back to 1877, when Jabłonna Nowa (New Jabłonna) rail station was built. The name comes from Jabłonna, the nearby village, where in 1774-1779 Bishop Michał Poniatowski (brother of Poland's last king, Stanisław Poniatowski) built his palace Jabłonna Palace (Polish: Pałac w Jabłonnie) - now owned by the Polish Academy of Sciences.
In 1892 the Russian army barracks (koszary carskie) were built near the railroad station (Obóz Hurki) and a local garrison of the Russian army was stationed there, as a part of Warsaw Stronghold Region (Warszawski Rejon Umocniony), until the beginning of World War I, when this region was occupied by the German troops.
In 1912, Legionowo is given a city rights (prawa miejskie).
In 1919, Jabłonna Nowa was renamed as Legionowo to honour Polish Legions (Legiony Polskie).
Ca. 1920 Institute of Aerology (currently Aerology Centre within Institute of Meteorology and Water Administration, Ośrodek Aerologii Instytutu Meteorologii i Gospodarki Wodnej) was opened.
During the Battle for Warsaw (Bitwa o Warszawę) in August 1920, from barracks in Legionowo, General Żeligowski led the 10th Infantry Division (10. Dywizja Piechoty) to Radzymin, which helped to save Warsaw from the Red Army.
In 1922 the Aviotex balloon and parachute factory, which also produced tents and other camping equipment (e.g. sleeping bags), opened in Legionowo.
After 1925, Legionowo became a summer resort for inhabitants of Warsaw, as by that time, it was a wooded and unpolluted area.
Between World War I and World War II, narrow-gauge railroad line connected Legionowo with Warsaw, going through Jabłonna.
In 1930 Legionowo became a commune (gmina).
During World War II a ward of Stalag 368 (German: Stammlager, prison camp for lower officers and soldiers) in Beniaminowo and a ghetto were located in Legionowo.
During World War II, in 1944, Legionowo took part in Warsaw uprising, as so called District 7: Collar (Obwód 7: Obroża). During the first week of August 1944, Legionowo was a place of regular fights between German troops and Polish rebels. After a week or so, Germans put down the uprising and several Poles where executed in one of the military shelters near the railroad line.
After World War II there was a brick factory (cegielnia), now non-existent.
In the 1950s, the standard-gauge railroad line was electrified.
In the late 1960s, narrow-gauge railroad line was closed and in early 1970s the tracks were removed. The terminus and depot buildings remain, currently in private use.
In the 1960s, the first 4-storey blocks of flats were built. In 1970s and 1980s, three large groups of flats (4-storey and 11-storey) were built.
In 1977 a tin ware factory "Bistyp" was opened.
In the early 1980s, a house factory was built near Legionowo, which made prefabricated elements for blocks of flats built in the region. The factory is now closed.
In August 1990 the Police Training Centre (Centrum Szkolenia Policji), one of two such institutions in Poland, was opened.
After the big flood in southern parts of Poland in 1997, the Aerology Institute was equipped with Doppler meteorological radar, able to scan about a fifth of Poland's area for storm and rain clouds. It is now part of SMOK (The Hydrological and Meteorological Monitoring Forecasting and Protection System, Polish: System Monitoringu i Osłony Kraju).
In 2012, Legionowo hosted the Greek football team during the Euro 2012 football competition.