Myasnikyan was the son of a merchant. He graduated from the law department of Moscow University in 1911. As a student in Nakhichevan and later in Moscow, Myasnikyan was active in underground groups beginning from 1901 and formally became a member of the revolutionary movement in 1904. He was arrested and exiled to Baku in 1906.
Between 1912 and 1914, Myasnikyan worked as an assistant to a lawyer in Moscow and participated in disseminating political literature. He was drafted into the Russian Army in 1914, where he promoted revolutionary ideas among the soldiers.
After the February Revolution of 1917, Myasnikyan became a member of the Western Front's frontline committee and was an editor of the Bolshevik newspaper Zvezda in Minsk. He was elected as a delegate for the 6th Congress of the Bolshevik Party. He became chairman of the Northwestern Regional Committee of the Bolshevik Party, and became part of the Military Revolutionary Committee of the Western Region. Myasnikyan was elected the commander of the Western Front at the congress of deputies.
Despite being an active opponent of the idea of a Belarusian autonomy, in 1918, he was appointed the first chairman of the Communist Party of Byelorussia. From 4–27 February 1919, Myasnikyan was chairman of the Central Executive Committee of the Socialist Soviet Republic of Byelorussia that briefly existed in January and February of that year. He was a member of the Central Committee of the Bolshevik Party for the short-lived Lithuanian–Belorussian Soviet Socialist Republic.