Weltzien attended the Mariengymnasium in Jever and joined the infantry regiment of the Grand Duchy of Oldenburg as a volunteer on 21 June 1829.

In 1839 he joined the recently founded Literary and Sociable Association and took an active part in the then flourishing movement of moderation.

A Major general since 30 April 1865, Weltzien was the successor of Wilhelm von Ranzow as commander of both the Oldenburg Contingent and the Oldenburg-Hanseatic Brigade.

In the next year he led his brigade in the Austro-Prussian War as part of the Army of the Main for the campaign against Austria's southern German allies.

When the German military was reorganized and the federal Oldenburg Contingent subsequently absorbed by the Prussian Army, Weltzien was attached to the staff of the 15th Division in Cologne on 25 September 1867.