By the time the Russian Empire entered World War I on 1 August 1914, Troyanov was a captain in the Sevsky Regiment.

On 20 April 1915, Troyanov was appointed commander of the Czechoslovak Legion, a volunteer force formed in the Russian Empire of former soldiers of the Austro-Hungarian Army of Czech and Slovak nationality who had switched sides to fight on the side of the Triple Entente against the Central Powers.

Promoted to colonel, Troyanov was able to strengthen the fighting ability of the Czech Legion′s soldiers and to win their love and respect.

Troyanov stayed on in the post-imperial Russian Army, and on 1–2 July 1917, under Troyanov′s command, the Czechoslovak Legion defeated numerically superior Austro-Hungarian forces in Galicia in the Battle of Zboriv during the Kerensky Offensive.

For his skillful leadership in the battle, Troyanov was promoted to major general and was made commander of the 1st Finland Rifle Division.