Karel Slabbaert was born in 1618/19 in Zierikzee to Carel Slabbaert and Catharina Goltzius from Bruges. In 1640 and 1641 he signed legal documents in Leiden, from which we learn that he was acquainted with the painters Pieter de Potter and Isaak Koedijck. From 1641 his activity can be followed on the basis of dated works. On 8 April 1645 he became engaged in Amsterdam to Cornelia Bouwers from Rotterdam. At that time he was living in Amsterdam, though in the same year he joined the painters' guild in Middelburg. In 1649/50 he was one of the guild's directors, and in 1653/54 he rose to the office of dean. He was buried in Middelburg on 6 November 1654.
Slabbaert appears to have been a sought-after portraitist in Middelburg. He not only painted portraits, but also made portrait engravings of the city's prominent citizens. High points in his little-known oeuvre are a few original genre compositions that are indebted to models from the Leiden School around Gerrit Dou. He also painted still lifes. In 1656 a large painting with a golden beaker in the Leiden collection of the painter Abraham de Pape was appraised at no less than sixty gulden.
Abraham Bastiaensen, Abraham Born and Eeuwoudt Adriaensz. van Schagen were his pupils in 1653/54.
