Janusz I was Duke of Warsaw (from 1373), one of the dukes of Masovia

Dukes of Masovia

The Dukes of Masovia were a line of the Piast dynasty who ruled in Masovia. The following is a list of all rulers of the Duchy of Masovia and its parts...

 after its division amongst the sons of Duke Siemowit III

Siemowit III of Masovia

Siemowit III of Masovia was a prince of Masovia and a co-regent of the lands of Warsaw, Czersk, Rawa, Gostynin and other parts of Masovia.- Life :...

.

Among the lands in his domain were the areas of Warsaw

Warsaw

Warsaw is the capital and largest city of Poland. It is located on the Vistula River, roughly from the Baltic Sea and from the Carpathian Mountains. Its population in 2010 was estimated at 1,716,855 residents with a greater metropolitan area of 2,631,902 residents, making Warsaw the 10th most...

, Nur

Nur, Poland

Nur is a village in Ostrów Mazowiecka County, Masovian Voivodeship, in east-central Poland. It is the seat of the gmina called Gmina Nur. It lies approximately south-east of Ostrów Mazowiecka and north-east of Warsaw....

, Łomża, Ciechanów

Ciechanów

Ciechanów is a town in north-central Poland with 45,900 inhabitants . It is situated in Masovian Voivodeship . It was previously the capital of Ciechanów Voivodeship.-History:The grad numbered approximately 3,000 armed men....

 and Różan

Rózan

Różan is a town in Mazovian Voivodeship, Poland. It is located at around . Narew River flows through the town....

. In 1374 he inherited Zakroczym

Zakroczym

Zakroczym is a small town in the Masovian Voivodeship, Poland. It is located at around . The Vistula River flows through the town....

 and in 1381 the lands of Czersk

Czersk

Czersk is a town in northern Poland in Chojnice County, Pomeranian Voivodeship.On July 1, 2006 this municipality celebrated 80 years of granting this community the status of city....

, Liw

Liw

Liw is a village in the administrative district of Gmina Liw, within Węgrów County, Masovian Voivodeship, in east-central Poland. It lies on the river Liwiec , approximately west of Węgrów and east of Warsaw....

 and Wyszogród

Wyszogród

Wyszogród is a town in Poland, in Masovian Voivodship, in Płock County, by the Vistula River. The population of Wyszogród was 2,793 in 2004.-History:...

. In 1376 he married princess Danutė

Danute of Lithuania

Danutė of Lithuania was a Lithuanian princess, daughter of Kęstutis, Grand Duke of Lithuania. She was married to Janusz I of Warsaw, one of the dukes of Masovia. Very little reliable information is known about her life.The date of her marriage is confused in many sources...

 (Danuta Anna), daughter of Lithuanian Duke Kęstutis

Kestutis

Kęstutis was monarch of medieval Lithuania. He was the Duke of Trakai and governed the Grand Duchy of Lithuania, 1342–82, together with his brother Algirdas , and with his nephew Jogaila...

. Because of his family ties, he served as a mediator between king of Poland Jogaila

Jogaila

Jogaila, later 'He is known under a number of names: ; ; . See also: Jogaila : names and titles. was Grand Duke of Lithuania , king consort of Kingdom of Poland , and sole King of Poland . He ruled in Lithuania from 1377, at first with his uncle Kęstutis...

 and the Grand Duke of Lithuania Vytautas.

In 1386 Janusz became a hereditary vassal

Vassal

A vassal or feudatory is a person who has entered into a mutual obligation to a lord or monarch in the context of the feudal system in medieval Europe. The obligations often included military support and mutual protection, in exchange for certain privileges, usually including the grant of land held...

 of Poland

Poland

Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...

. In 1391 he inherited the title of the Duke of Podlaskie. In 1406 he moved the capital of Masovia from Czersk to Warsaw. In 1409 he organised a banner of cavalrymen to aid his liege, the king of Poland, in the war against the Teutonic Order. The following year together with his banner, despite his age, he took part in the Battle of Grunwald

Battle of Grunwald

The Battle of Grunwald or 1st Battle of Tannenberg was fought on 15 July 1410, during the Polish–Lithuanian–Teutonic War. The alliance of the Kingdom of Poland and the Grand Duchy of Lithuania, led respectively by King Jogaila and Grand Duke Vytautas , decisively defeated the Teutonic Knights, led...

.

With Danuta, he fathered three sons: Janusz Młodszy (Janusz the Younger), Bolesław Januszowic and Konrad Januszowic. He outlived them all, and passed his territories to his grandson, Bolesław IV of Warsaw.