Due to the scarcity of documentary evidence, his place of birth and nationality are a matter of dispute between modern Lithuanian and Belarusian researchers.
The Lithuanian scientific school asserts that he was born near Raseiniai in Samogitia. The family, who was relatively poor, bore the Ostoja Coat of Arms with military service traditions in the Grand Duchy. In a book dedication, he refers to himself as an "Eques Lithuanus" (Lithuanian nobleman). Siemenowicz was educated in the Academy of Vilnius.
The Polish school, instead of defining his ethnicity, describes his identity simply as member of the szlachta (i.e., nobility in the Commonwealth) from Grand Duchy. Through some sources use the term "Polish," others describe him as "Lithuanian." Those terms should be understood in proper context: "Polish" means "of the Polish–Lithuanian Commonwealth"; "Lithuanian" from the Grand Duchy of Lithuania, a federal part of the Commonwealth. Polish historian Tadeusz Nowak described Siemienowicz as a Polonized Lithuanian nobleman. Polish historians for the most part accept that he used the Ostoja Coat of Arms and that he was an alumnus of the Academy of Vilnius.
The Belarusian school asserts that he was born in the vicinity of Dubrowna in the Vitsyebsk land, to a family of minor Ruthenian princes (knyaz) of Siemienowicz, who possessed the small tracts of land in that part of the Belarusian Dnieper-land (Падняпроўе) in the 14th–17th centuries. Some examples of lexicography used by K. Siemienowicz support this interpretation.
There are no records of families with surname Siemienowicz having the right to bear the Ostoja coat of arms and it is possible that Siemienowicz acquired the right to use the image of Ostoja in his book to facilitate its circulation. He, however, was vague in his autobiographical notes as to his position in the hierarchy of nobility, possibly because of that. The Belarusian school of the 20th century tends to interpret the "Litvin/Lithuanian" denomination of the Medieval/Renaissance period as a politonym rather than ethnonym; when Siemienowicz writes about himself as "Litvin or Lithuanian," he means a citizen of Great Duchy of Lithuania, not ethnicity. The identity of Siemienowicz and the magister of philosophy and liberal arts of same surname recorded in the acts of Vilnius academia of 1650 is disputed.
As Siemienowicz wrote, he was fascinated by artillery since childhood, and he studied many sciences to increase his knowledge (mathematics, mechanics, hydraulics, architecture, optics, tactics). In 1632–1634 he took part in the Smolensk War, in the Siege of Belaya under Mikołaj Abramowicz (who in 1640 became the first Lithuanian General of Artillery). It is possible that in 1644 he took part in the Battle of Ochmatów.
He spent some time in the Netherlands, where he was sent by the King Władysław IV Vasa to serve in the army of Duke Frederick Henry of Orange during the war with Spain; he participated in the Siege of Hulst in 1645. In 1646 he returned to Poland, when Władysław created the Polish artillery corps and gathered specialists from Europe, planning a war with Ottoman Empire. He served as an engineering expert in the fields of artillery and rocketry in the royal artillery forces. From 1648 he served as Second in Command of the Polish Royal Artillery.
In late 1648 the newly elected king John II Casimir Vasa, who had no plans for the war with Ottomans, advised him to return to the Netherlands and publish his studies there. There are rumors that in 1649 Siemienowicz became embroiled in a conflict with General of the Artillery Krzysztof Arciszewski over a bureaucratic matter; around 1649 he decided to leave the Commonwealth and work on his book in Amsterdam.
Siemienowicz considered the use of poison gases dishonorable. In his work, he wrote:
"and most of all, they shall not construct any poisoned globes, nor other sorts of pyrobolic inventions, in which he shall introduce no poison whatsoever, besides which, they shall never employ them for the ruin and destruction of men, because the first inventors of our art thought such actions as unjust among themselves as unworthy of a man of heart and a real soldier.
In a historically early instance of biowarfare, Siemienowicz sponsored the firing of artillery containing the saliva of rabid dogs during a 1650 battle. While the success of this experiment is unknown, it demonstrated an educated guess about the disease's communicability that was not confirmed until the 18th century.
In 1650 Siemienowicz published a notable work, Artis Magnae Artilleriae pars prima (Great Art of Artillery, the First Part). Its name implies a second part, and it is rumored that he wrote its manuscript before his death. It is also rumored that he was killed by members of the metallurgy/gunsmith/pyrotechnics guilds, who were opposed to him publishing a book about their secrets, and that they hid or destroyed the manuscript of the second part. Siemienowicz disparaged what he saw as a culture of secrecy based on "canting Alchymists of the times Past...they dealed [sic] in nothing but Smoke, yet arrogantly took upon them to be Professors of so noble and excellent an art as Chymistry."
Artis Magnae Artilleriae pars prima was first printed in Amsterdam in 1650, was translated to French in 1651, German in 1676, English and Dutch in 1729, and Polish in 1963.
In the first part of his work he wrote that the second one would contain the "universal pyrotechnic invention, containing all of our current knowledge." According to his short description, this invention was supposed to greatly ease all measurements and calculations.
For over two centuries this work was used in Europe as a basic artillery manual. Its pyrotechnic formulations were used for over a century. The book provided the standard designs for creating rockets, fireballs, and other pyrotechnic devices. It discussed for the first time the idea of applying a reactive technique to artillery. It contains a large chapter on caliber, construction, production and properties of rockets (for military and civil purposes), including multistage rockets, batteries of rockets, and rockets with delta wing stabilizers (instead of the common guiding rods).