Grondijs was born in the Dutch East-Indies, now known as Indonesia, to an Indo mother. He spent most of his youth in the East Indies and graduated in 1896 from grammar school in Surabaya. A gifted academic he continued to study in the Netherlands. He graduated in mathematics and physics at Utrecht University in 1905 and continued his studies in philosophy and mathematics at Leiden University. In 1907 with J.D. Bierens de Haan he founded the Journal of Philosophy Tijdschrift voor Wijsbegeerte and in the 1930s he became a leading expert in Byzantology.
Working as a teacher at the Dordrecht Technical Institute in 1914, he quit his post when the Great War broke out and secured a position as war-correspondent for the Dutch newspaper Nieuwe Rotterdamsche Courant. He went into neighboring Belgium where he covered the early events of the war in Aerschot, the German war crimes at Leuven as well as the siege of Antwerp. He published a book on his experiences in Belgium, The Germans in Belgium - Notes by a Dutch Eye-Witness and afterwards traveled to France, working as a war-correspondent for various international newspapers and news-magazines. For saving fifty Belgian clergymen from German execution during the Rape of Belgium period, including the rectormagnificus of the famous Catholic University of Leuven, he was decorated officer in the Belgian Order of the Crown.
Later in September 1915, he left for Russia at the invitation of general Aleksei Brusilov where he was allowed to accompany the Russian 8th Army as a correspondent of The Daily Telegraph. Many of his vividly written articles on warfare on the Eastern Front were published in the prestigious French newsweekly l'Illustration. He apparently respected the fighting qualities of the common Russian soldier and expressed his admiration numerous times in his articles. And although an academic by profession, he seemed to relish the adventure and excitement of war-time journalism and of warfare itself; he is said to have taken active part in combat along with his Russian hosts on many occasions. For this, he was decorated with the Imperial Russian Order of St. George, Order of St. Stanislaus, Order of St. Anna and Order of St. Vladimir.
He was present in Petrograd during the initial February Revolution in 1917, but after the Bolshevist take-over, he left for White controlled territory where he joined counter-revolutionary armies of generals Lavr Kornilov and Mikhail Alekseev and reported on the Russian Civil War. In June 1918 he was the only western war correspondent to join the Volunteer Army in the Kuban Campaign. Oddly he appears to have found the time to obtain a doctor's degree in physics at the university of Charkov in 1917 on the thesisElektromagnetische Feldgleichungen bewegter Systeme. In 1918 he became an accredited war-correspondent to the French government, for which later that year he travelled to the USA, Japan and the Russian Far East.