[1] On 1 April 1911, he joined the Imperial German Navy as an officer candidate where he was put in charge of a flying boat squadron during World War I.

After the disbandment of the imperial military in 1919, von Gronau had reached the rank of Oberleutnant zur See, highest lieutenant grade in the German Navy.

He took off from Sylt (Germany) through Faroe Islands,[2] Iceland, Greenland and Labrador, reaching New York City after covering 7,520 km (4,670 mi) in 47 flight hours.

He returned 111 days later, on 10 November, after having taken off from List auf Sylt westwards across Iceland, Greenland, Canada, the Aleutians, Alaska, the Kurils, Japan, China, the Philippines, Indonesia, Malacca, Burma, Ceylon, India, Iran, Iraq, Cyprus, Greece and Italy, landing finally in the Lake of Constance after having covered over 44,000 km (27,000 mi).

Von Gronau was posted to the German Embassy in Tokyo as air attaché shortly before the outbreak of World War II.