The lands on which Kosów Lacki is situated, i.e. the historical border between Mazovia and Podlachia (Podlasie in Polish), since the earliest times have been a colorful mosaic of cultures and social communities. These lands were inhabited – to give just a few most significant examples – by the Poles who spoke Masovian dialects, by descendants of the Yotvingians, Lithuanians and Ruthenians who spoke dialects with certain linguistic traits and peculiarities of Belarusian and Ukrainian languages, and the Poleshuk dialect, by the Jews, Germans, Tatars, Scots, Dutch, and the Great Russians – Old Believers. The experiences, religions, customs and languages brought by them have merged into a multi-coloured image of the Podlachia culture.
Kosów appeared in history 200 years later: in 1417, on the first Sunday after Corpus Christi, the Grand Duke of Lithuania, Vytautas, granted Marcin of Ciołków, District Mayor (starosta in Polish) of Pułtusk, with estates in Podlachia, including the villages of Grądy, Nowa Wieś, Łomna, Olszew, Lubiesza, Dębe, Guty, Golanki and Kosów. In 1425, Marcin of Ciołków built the first church here and founded the parish of Virgin Mary and St. Joseph. In the early sixteenth century, the Kossowski family founded a second church here, i.e., the Orthodox Church of Theotokos (Mother of God), situated on the so-called "Ruthenian" side. As a result of the Union of Brest (1596), it became a Uniate church within the authority of the Drohiczyn deanery in the Brest diocese.
It was customary to have a hospital at the church; it was a poorhouse that looked after and cared for the poor and needy who were called "beggars".
After the turmoil of the Swedish invasion and wars in the seventeenth century, in order to save the depopulated and destroyed lands, the Kossowski family brought the Jews, who initiated trade and started local businesses. In 1723, Kosów was named “the town” in church documents (oppidum in Latin).
After the third partition of Poland, Kosów came under the Austrian rule, which changed little for its residents. In the nineteenth century, with the emergence of the Duchy of Warsaw (1807) and the Napoleonic Code coming into effect as well as new administrative divisions, local authorities were established in the town. The first mayor, as was customary, was the owner of the town, Jacek Paderewski.
In 1820, a church, a synagogue and 26 wooden houses arranged around a rectangular unpaved market square were present in Kosów; next to the market there were taverns, butcheries, two inns, a distillery and a tannery. A part of Kosów as the town settlement (today's Wolności Street and adjacent streets) was inhabited mainly by the Jews, the remaining parts – by Christians. The proportion of this number varied. In the nineteenth century, there were two impressive wooden churches in the town: a church and a synagogue.
In July 1870, the Organizing Committee in the Kingdom of Poland decided to change the urban status of Kosów (and Sterdyń) and make it a village. It did not significantly change the functioning of the place itself or of its inhabitants. Since the turmoil of the January Uprising in 1863, it has not suffered any major tragedies. It was not until August 1915, when the storm of the Great War broke out, that this peace was disturbed. After regaining independence and following the fights fought in and around Kosów in August 1920, this area began to recover. Libraries, an amateur theatre, paramilitary organisations, a number of party representations - both Polish and Jewish, and social organisations dealing with combating poverty were established in Kosów. Life flourished, and the town was given a new face as artesian wells and sanitary facilities were built, a new school was erected, streets were paved, thr monument to Marshal Józef Piłsudski was erected, and 3 May Constitution Day and the Independence Day were celebrated each year.
World War II began here in the second decade of September with a demonstrative march of the German troops through the main street of Kosów. The occupiers took over the school building, the Brodzikowski family’s mansion and some more elegant buildings, too. A ghetto was set up and the Jews from various parts of Poland were relocated there. When the Treblinka death camp was established in 1942, the ghetto was liquidated, and all its inhabitants were driven on foot to their death in gas chambers.
During World War II, a penal labour camp, commonly known as Treblinka I, was established in the forests near Wólka Okrąglik (in the commune of Kosów Lacki). The name was associated with the nearest railway station, which was in the village of Treblinka. The establishment of the camp (at the previously existing gravel pit) was the initiative of Ernst Gramss, District Mayor (starosta in Polish) of Sokołów Podlaski. The camp operated from the summer of 1941 to late July 1944. People were sentenced to stay there; initially, they were residents of the Sokołów district, and were punished for various offences and misdemeanours against the occupier. Over 20,000 prisoners passed through the camp, of whom about 10,000 died or were shot.
The death camp Treblinka II was built by the Germans in mid-1942. It was established as part of Operation Reinhard that was aimed at the extermination of the Jewish population. It occupied 17 hectares of land and was surrounded by a high barbed wire fence. For most of the camp's operation, its commandant was Franz Stangl, and his deputy was Kurt Franz. The latter documented the operation of the camp in photographs, and the resulting album was entitled "Beautiful Times".
The first transport of people arrived on 23 July 1942 and brought the Jews from the Warsaw ghetto. From that day onwards, the Jews were brought here mainly from occupied Poland, but also from Czechoslovakia, France, Greece, Yugoslavia, the USSR, as well as Germany and Austria. Roma and Sinti from Poland and Germany were also transported here. The victims were killed in gas chambers and their bodies were burned on specially constructed grates.
On 2 August 1943, an armed prisoners’ revolt broke out in the camp. Out of 840 people staying there, only about 200 managed to escape. About 100 of them stayed there until the end. After the revolt, the camp was gradually liquidated. In 1943, all the camp buildings were dismantled and a house for a Ukrainian family was built in their place. The area of the camp was ploughed and sown with lupin. Before the advent of the Eastern Front, the buildings were burned down.
Over 800,000 people died here.
After the so-called “liberation”, the Red Army stayed in the town and its vicinity for a long time. Until the 1950s., pro-independence guerilla forces fought the new occupant.
In 1964, in the presbytery in Kosów Lacki, Izabella Galicka and Hanna Sygietyńska accidentally discovered the only painting in Poland - "The Ecstasy of Saint Francis" - the authorship of which is attributed to a Spanish painter of Greek origin, Dominikos Theotokopulos, known as El Greco.
In 2000, Kosów, as part of the new Masovian Voivodeship, received its town charter. It is one of the two towns in the Sokołów District.