[2] At the age of sixteen he joined the Bar Confederates, and served as a secretary to Michał Wielhorski, accompanying him on his diplomatic mission to Paris, France, in 1770.

[3] Zajączek stayed in Paris for several years, and in 1773 he met one of the major leaders of the Confederacy, Casimir Pulaski.

[6] Thanks to the support from the magnate Sapieha family he received a post of an aide-de-camp to hetman Franciszek Ksawery Branicki.

[19] He took part in some minor skirmishes, but did not participate in any significant battle before Stanisław August Poniatowski surrendered to the enemy, joining the Targowica Confederation.

[20] The war ended without any decisive battles, with the Polish army still in the fighting condition, not suffering from any major defeat nor from lack of supplies.

[20] Like many other dissatisfied officers, including Prince Poniatowski and Tadeusz Kościuszko, Zajączek requested a discharge from the army; his resignation was accepted on 18 August.

[20][23] In August 1793 he moved to Leipzig, joining a number of other Polish emigres, namely Kościuszko and Kołlątaj, preparing for a new war.

[24] Soon he became one of the major planners of this approaching conflict, often tasked with delivering important messages to Poland, and collecting information there; in particular he was active in Warsaw, and Kościuszko planned for him to become a commander of that region.

[28] Zajączek was given the command of the Lesser Poland Division, and on 4 April he took part in battle of Racławice, which concluded with a major Polish victory.

[38] Hoping to sway revolutionary France into supporting the Polish cause, many Poles, including him, volunteered to join the French Revolutionary Army, eventually leading to the creation of the Polish Legions in Italy led by Jan Henryk Dąbrowski, which became a part of Napoleon's army.

[39] On 8 March 1797 Napoleon ordered that Zajączek should be recognized as an active brigade general of the French army.

[51][52] During that time, dissatisfied with his transfer from the French army, and being made subject to Prince Józef Poniatowski whom he disliked, he refused to wear a Polish uniform and swear an oath to the Duchy, and was forced to do so only on direct orders of French marshal Louis-Nicolas Davout.

[53][54] In 1809 he commanded several formations in the Polish–Austrian War,[55] where he failed to distinguish himself, instead suffering the only significant defeat in this campaign at the battle of Jedlińsk on 11 June.

[56] He remained in the Army after the war, commanding the Polish 2nd Division, and organizing the troops for the expected conflict with Russia.

[57] During Napoleon's Invasion of Russia in 1812, Zajączek commanded the 16th Infantry Division of the V Corps in la Grande Armée.

[68][69] This nomination surprised many, including Zajączek himself, who is said to have at first refused this position, but in the end accepted it after Alexander phrased his request as a direct order.

[73] He has been criticized, however, for being inexperienced for such a major civil leader position,[74] too servile to the wishes of his patron, Grand Duke Constantine Pavlovich[75] and he often disregarded the Constitution of the Congress Poland, which contributed to a slow but steady erosion of the country's freedoms and autonomy.

[76] Deferential towards the Russian authorities, he readily let Duke Constantine and Alexander's commissar, Nikolay Nikolayevich Novosiltsev, neither of whose positions were envisioned in the Constitution, become the de facto rulers of the Kingdom.

[81] He was buried at the Church of the Holy Heart of Jesus in Opatówek, where he had a manor on the lands received from Napoleon.