Sakellariou entered the Hellenic Naval Academy in 1902 and graduated in 1906. He served in the Royal Hellenic Navy in the Balkan Wars and until 1918, when he was discharged due to his royalist convictions during the National Schism. As with many other royalist officers, he rejoined the military in the 1920s. In March 1935, as Rear Admiral, he commanded the naval forces that were dispatched to subdue the naval units that took part in the Venizelist coup attempt. After the coup's suppression, he served as president of the court martial that tried the mutinous officers, and was promoted to Chief of the Fleet Command.