He was born in Batavia, Java, Dutch East Indies, and also died there. He was the son of a Dutch father Pieter Jacobus Adrianus Zaalberg, Secretary at the Department of Education and Religious Affairs in the Dutch East Indies, and Indo (Eurasian) mother Susanna Elisabeth de Bie. He was married to Maria Taunay (born 1878) from 22 April 1899 up to her death in 1911. Both the Zaalberg and De Bie family were of Jewish descent. They had two sons and three daughters, one of whom died young.
Karel Zaalberg was a self-taught and self-made man. With only primary school he became chief editor of one of the biggest newspapers. Education for the Indo-Europeans of the Dutch East Indies would be his life’s personal and political ambition. As a journalist he became a spokesman for the Indo-European (Eurasian) community.
He was director of the first organization for Indo-Europeans in the Dutch East Indies (‘Indische Bond’ of 1898). After 1919 he became a founder and board member of the Indo-European Alliance (Dutch: ‘Indo Europeesch Verbond'), with over 10,000 members the largest interest group of this population. From 1924 until his death, four years later, he represented the party in the Dutch East Indies House of Assembly (Dutch: Volksraad). 
Zaalberg was a leading figure in the political and social emancipation of the Indo (Eurasian) community of the Dutch East Indies of the late 19th and early 20th century, who had closely befriended famous figures of the time like writer P.A. Daum, E.du Perron and independence activist Ernest Douwes Dekker.