(also Cornelius Jonson van Ceulen, Cornelius Johnson, Cornelis Jansz. van Ceulen and many other variants)(bapt. October 14, 1593, London - bur. August 5, 1661, Utrecht) was an English painter of portraits of Dutch or Flemish parentage. He has been described as "one of the most gifted and prolific portrait painters practising in England during the 1620s and 1630s".
Janssens van Ceulen was born to Dutch or Flemish parents in London - his father had been a refugee from Antwerp, and the family had originated in Cologne. He was baptised at the Dutch church at Austin Friars, the son of Johanna le Grand and Cornelius Johnson. He may have been trained in the Netherlands, and was certainly influenced by other artists from the Netherlands, but he was active in England, at least from 1618 to 1643. In the 1620s, he lived and had his studio in Blackfriars, London, as did Anthony van Dyck; it was just outside the boundaries of the City of London, and so avoided the monopoly in the City of members of the London painters' Guild. He married Elizabeth Beke of Colchester in 1622. Janssens' son (Cornelius Janssens, junior) was born in 1634. He was also a painter. Janssens' daughter was married to Nicholas Russell of Bruges. Janssens moved to Canterbury in the mid 1630s, living with Sir Arnold Braems, a Flemish merchant. Janssens continued to live in England until after the outbreak of the English Civil War, but in October 1643, apparently at the insistence of his wife, he moved to Middelburg, and between 1646 and 1652 he lived in Amsterdam, before settling in Utrecht, where he was buried.