The Matsuyama claimed descent from a retainer of the medieval pro-Imperial hero Kusunoki Masashige, and as a youth Yashiro joined a cadet movement of pro-Sonnō jōi militia of Owari Domain and despite his young age, fought in the Boshin War.

After his return to Japan, he was assigned to the cruiser Takachiho, and was serving as a division officer aboard that vessel during the Battle of the Yalu on 17 September 1894 in the First Sino-Japanese War.

Yashiro briefly served on the cruiser Yoshino during the next year, before being assigned as a naval attaché to Saint Petersburg, Russia, where his principal duty was making an Intelligence assessment of the capabilities of the Imperial Russian Navy.

Yashiro developed the plan to blockade the entrance to Port Arhur by sinking obsolete ships: however, he was overruled in his desire to lead the attack in person by Admiral Tōgō Heihachirō.

The opportunities for Japan to use the Anglo-Japanese Alliance as an excuse to enter the war and to expel Germany from Asia, seizing its possessions in the Shantung Peninsula and in the islands of the South Pacific were clear, but Yashiro was at best ambivalent, knowing how these actions would create a larger conflict with the United States.

[2] Although appointed due to his commendable military and apolitical record, Yashiro's bluntness over issues with the Navy's budget led to his forced resignation in April 1915.