Petrova fought in the Soviet-Finnish War before volunteering for service in the German-Soviet War. Initially she served in the 4th Division of the Leningrad People's Militia before entering a medical battalion, even though she was too old to be drafted. In November 1941 she transferred to fight as a sniper in the 1st Infantry Battalion of the 284th Infantry Regiment of the 86th Tartu Rifle Division, where she rose through the ranks to sergeant-major (starshina). In the Battle of Leningrad she trained other soldiers of the Red Army as snipers in addition to participating in direct combat.