The diocese museum in Siedlce was established in 1918, which was the year when Pope Benedict XV restored, after annexation, the Janów Podlaska Diocese.
At that time the Ordinary was Bishop Henryk Przeździecki, who founded the Museum that operates until today. The first administrator of the Museum was Father Karol Dębiński, the Doctor of the Ecclesiastic Academy in Petersburg and the Professor of this university before the soviet revolution, the official of the bishop`s Court in Janów Podlaski and in Siedlce, vicar general of the Curia and the founder of the Bishop`s Secondary School in Siedlce.
In 1919 the Government commissioned the first physical inventory of artifacts but until today the only part of it that survived are a list of coins and a few notes about some exhibits.
In 1922 the Museum collections were relocated to Siedlce and the exhibition was displayed in the Curia building.
During World War II 1939-1945 German occupation forces plundered the Museum and closed it but fortunately a part of the most precious exhibits was hidden by Father Jan Grabowski.
In 1974 Jan Mazur, the bishop of Siedlce appointed a chairman of the Diocese Artistic Architectural Commission, W. Skomorucha, the associate bishop, as well as founded the office of the Diocese Monuments Conservator managed by Father Tadeusz Kulik. The task of these two institutions was i.a. the collection of works of art no longer used as objects of cult, in order to protect the cultural heritage against destruction dispersal or alienation.
The period from 1974 to 1991 was the time when the institution commissioned the conservation of 125 most precious paintings on wood and canvas, of wooden sculptures, manuscripts and fabrics conservation. Works were performed by professionals from PKZ and private conservation workshops from Warsaw, Toruń, Kraków, Gdańsk and Siedlce.
Since 1991 the Museum has occupied the building at 56 Bishop Ignacy Świrski Street. Works related to adaptation and organisation of the new venue lasted until 2000, which was also a year of death of Father Tadeusz Kulik, the first postwar administrator of the Museum. In the same year Bishop Jan Wiktor Nowak founded the Diocese Museum and appointed as its director Father Henryk Drozd, a historian of art. The official opening took place on June 1st 2000.
An important event that followed in 2004 was the presentation of the only El Greco`s painting in Poland „The ecstasy of St. Franci`s” on October 15th. Bishop Zbigniew Kiernikowski expressed his gratitude to Father Henryk Drozd for his contribution in the aspect of the Museum`s opening and functioning, and appointed a new director, Father Robert Mirończuk, a phd in the history of art.