The oldest traces of settlement in the area of Przasnysz come from the turn of the Bronze and Iron Age (around 700 BC).

The name of the city according to folk sources comes from the miller Przaśnik, who hosted the stray hunting Duke Konrad I of Masovia and was then knighted with the surrounding lands.

Przasnysz's rapid development was due to its favorable location on the border between two economically important areas - the Kurpiowska Plain and the agricultural Ciechanowska Upland.

On 10 October 1427 Przasnysz obtained town privileges under the Chełmno law from the Masovian Duke Janusz I of Warsaw.

After the defeat of the Kościuszko Uprising and the Third Partition of Poland (1795), Przasnysz became part of the Kingdom of Prussia as the seat of a large county including Ciechanów.

In November 1863, Przasnysz was the site of a Russian execution of Stefan Cielecki [pl], commander of a Polish insurgent unit, which fought in northern Masovia during the January Uprising.

During World War I, in November and December 1914, heavy fighting took place near Przasnysz between the Russian and German armies.

On 21 August Przasnysz was liberated by the 202 Infantry Regiment of the Volunteer Division of Colonel Adam Koc.

It immediately carried out mass searches of Polish offices, courts and organizations, arrested dozens of Poles, and expelled 70 Jews.

On 3–4 December 1940 the German gendarmerie expelled around 500 Poles, including old, ill and disabled people, who were then held for several days in a camp in Działdowo and afterwards deported in freight trains to the Kraków District of the General Government.

[13] Shortly after the Soviet Army seized the city on 18 January 1945, the NKVD began mass arrests and deportations of Polish patriots.

The coat of arms of Przasnysz depicts a defensive wall made of red brick on a silver (white) shield, with three such towers of equal height.