Documents indicate that Cornelis Pietersz Bega was born in either 1631 or 1632 in Haarlem. His grandfather was the renowned history painter Cornelis Cornelisz van Haarlem (Dutch, 1562 - 1638), and his extended family included many other more minor painters, sculptors, and craftsmen. In the spring of 1653 he traveled with the portraitist and still life painter Vincent Laurensz van der Vinne (1628–1702) through Germany, visiting Frankfurt am Main, Heidelberg, Strasbourg, and Basel. Although, like Van der Vinne, he may have intended to continue on to Rome, Bega instead returned to Haarlem in June of that year. He joined the Guild of Saint Luke in 1654. Bega died unmarried in 1664 at the young age of 32, likely a victim of the plague that claimed many lives in Haarlem that year.
Dutch artist biographer Arnold Houbraken wrote that Bega was the “first and best pupil” of Adriaen van Ostade (Dutch, 1610 - 1685), a painter of low-life genre scenes. Bega similarly specialized in paintings of jovial taverngoers, smokers, quack doctors, alchemists, and musicians, but he also created tender and sympathetic images of peasant families and nursing mothers. Like Ostade, he also produced etchings, and he may have been one of the first Dutch artists to experiment with monotypes. Bega was also a confident draughtsman, and made numerous chalk drawings remarkable for their sculptural quality. Houbraken wrote that Bega and his close friend Leendert van der Cooghen (1632–1681) frequently drew from life together.
Early in his career, Bega painted in a loose, rough style similar to Ostade, although his paintings differ from Ostade’s in that they exhibit a greater degree of monumentality and occasionally include classical elements and figures derived from live models. Bega may have been influenced by the classicizing tradition in Haarlem painting, which he would have known both from the extensive art collection he inherited from his grandfather, as well as from contemporary Haarlem painters who had trained there, such as Salomon (1597–1664) and Jan de Bray (Dutch, c. 1627 - 1688). In the late 1650s, Bega began producing smaller paintings with fewer figures and highly detailed brushwork, perhaps influenced by Leiden fijnschilders (fine painters) such as Gerrit Dou (Dutch, 1613 - 1675) and Frans van Mieris (Dutch, 1635 - 1681). His attentive rendering of materials, along with the emotional sensitivity he brought to his subjects would influence later genre painters like Thomas Wyck (Dutch, c. 1616 - 1677), Jan Steen (Dutch, 1625/1626 - 1679), and Cornelis Dusart (Dutch, 1660 - 1704).
 A notarized document from April 16, 1650 states that Bega was 18 years old; see Neeltje Köhler and Pieter Biesboer, Painting in Haarlem 1500–1850: The Collection of the Frans Hals Museum, (Ghent, 2006), 100–101.
 Van der Vinne documented this trip, and Bega’s participation in it, in an illustrated diary now in the Haarlem Gemeentearchief (Stell. 21D/171–2 and Stell. 21D/231).
 Hessel Miedema, De archiefsbescheiden van het St. Lukasgilde te Haarlem 1497-1789, 2 vols., (Alphen aan de Rijn, 1980), 2:638.
 Houbraken tells a touching, if unverifiable, story of the death of Bega’s sweetheart from the same plague. Bega longed to embrace her while she lay dying, but his mother and the doctor had forbid him to touch her. Instead, Bega held out the handle of a broom to her, kissing one end she girl kissed the other. Alas, this sanitary measure would not save Bega from contracting the same disease; Houbraken, Arnold. De Groote Schouburgh der Nederlandtsche Kosntschilders en Schilderessen, 3 in 1 vols., The Hague, 1753. (Reprint: Amsterdam, 1976), 1:349–50.