Jansen was born in The Hague in approximately 1580. His parents were Hans Martens (who may have had the occupation of a peddler) and Maeyken Meertens, both probably from Antwerp, Belgium. He grew up with his sister Sara in Middleburg, at the time the second most important city of the Netherlands He was known as "street seller" who was constantly in trouble with the local authorities and later became a spectacle-maker.
He himself stated he was born in The Hague on the marriage file of his first marriage, with Catharina de Haene, on October 23, 1610. When this file was refound by Cornelis de Waard in 1906, De Waard found the following excerpt: Sacharias Jansen, j.g. uut Den Haghe, translated into modern English: Zacharias Jansen, bachelor from The Hague Before, it was often thought Jansen actually was a native of Middleburg. In 1612 Zacharias and Catharina had a son they named Zaccharias.
Following the death of Jansen's first wife in 1624, he married Anna Couget from Antwerp, who was the widow of some Willem Jansen (probably a family member of Jansen). He moved to Amsterdam in November 1626. He must have died before 1632, since his son Johannes declared his parents had died by the time he married in April 1632.
 InventionsBy choosing the profession of spectacle-maker, Zacharias Jansen entered a very competitive and secretive trade. These factors may have played a role in Jansen's claims of invention, especially relating to the telescope since he and Middelburg spectacle maker Hans Lippershey were direct competitors who practically lived next door to each other.
 MicroscopeJansen is associated with invention of the a single-lens (simple) optical microscope and the compound (2 or more lens) 9x magnification optical microscope, probably with the help of his father in 1595 while trying to find a way to make magnification even greater, to help people with seriously poor eyesight. Jansen's attribution to these discoveries is debatable since there is no concrete evidence as to the actual inventor. Also Jansen's date of birth may be as late a 1590 making him 5 years old at the claimed time of invention.
 TelescopeZacharias Jansen is also one of three people generally associated with the invention of the telescope in the Netherlands in 1608. That year Hans Lippershey filed the first known patent for the device on October 2 with the States-General of the Netherlands, followed a few weeks later by a second patent application by Jacob Metius of Alkmaar. Both were turned down because there were counter claims for the invention.
There are varying accounts and debate as to whether Jansen was actually the inventor of the telescope. German astronomer Simon Marius wrote an account of his patron Johan Philip Fuchs von Bimbach meeting an un-named Dutchman at the 1608 Autumn Frankfurt Fair (which ran the month of September) who tried to sell him a device that sounded like a telescope. Given his history as a street seller, this un-named Dutchman could have been Zacharias Jansen, which means he had a telescope at least a month before Lippershey's October 2, 1608 patent date. William de Boreel, who visited Middelburg to research the invention in 1655, interviewed Jansen's son and daughter. Boreel concluded that Jansen's telescope was finished about 1610. His research was referenced by Pierre Borel in De vero telescopii inventore. There are other claims Zacharias Jansen constructed the first telescope in 1604, or even earlier. Zaccharias Jansen son Zaccharias testified under oath that Hans Lippershey had stolen his father's invention of the telescope.
The claim that Jansen invented the telescope was not well-known outside the Netherlands. Lippershey's claim to the invention was the most prominent since he had the first recorded patent and strongly promoted it. Whilst that news was spread across the world in just a few months (eventually Galileo Galilei heard about it), several investigations on the subject in the 17th, 18th, 19th and 20th century, stating Jansen might have invented the device, were only published and read in the Netherlands. To make it even worse, just one of the investigations was published in English and just that one stated Lippershey did invent the telescope, making Jansen even more infamous in countries outside the Netherlands.
 Illegal activitiesIn the years 1613-1619, Jansen was tried several times for counterfeiting coins. Jansen grew up right next to the Middleburg mint where his brother-in-law worked. These circumstances made it very easy for Jansen to mimic the process of manufacturing money. He fled to the neighbouring village of Arnemuiden to avoid the high penalties for counterfeiting coins.
However, he continued counterfeiting coins in Arnemuiden. In 1619 he was apprehended for owning several devices he counterfeited coins with. Normally, one would have been sentenced to death for this crime. However, since the father of the Arnemuiden bailiff was found to be an accessory, it turned out better for Jansen. Thanks to this, the process was delayed to such an extent that Janssen was able to flee yet another time. Eventually, the case was dismissed. Jansen returned to Middleburg in 1621.
 Historical recordZacharias Jansen's life was documented by the many investigations on the subject before the Second World War. Many of the Middelburg archives were destroyed by a devastating bombardment on May 17, 1940, during the Nazi invasion of the Netherlands. If there had never been profound investigations, very little would be known of Jansen's life at all, since all files were lost in the fires following the bombardment.