At the age of 19, he married Sophia Sasbout, and went to Delft to study Greek with Petrus Resenius, rector of the Latin school.
He was studying civil and canon law at Leuven University when the Dutch Revolt broke out, prompting him to switch to theology.
After the murder of Cornelis Musius, Opmeer offered his services to the Army of Flanders and was appointed secretary to the Council of War by Francisco de Valdez.
After the Siege of Leiden, he returned to his native Amsterdam, until the city was lost to the rebels in 1578 and Catholic worship was prohibited.
He then moved to Delft and lived there the rest of his life, dying on 4 November 1594, and being buried in the Oude Kerk.