He was born in the village of Santa near Trapezus, Pontus. He studied medicine at the universities of Moscow and Odessa, graduating in 1910 and settling in the city Sochoum of Caucasus. In 1913, he travelled to Germany for post-graduate studies. He specialised as a surgeon in Odessa, taking the position of the director of the hospital of Sochoum. After the declaration of independence of Georgia, he was elected a deputy with Noe Zhordania's Georgian Social Democratic Labour Party in the February 1919 general election.
In 1922, Passalidis settled in Thessaloniki and worked as a doctor. In the 1923 general election he was elected for the first time to the Hellenic Parliament as a republican deputy. In 1941 his party formed an alliance with EAM-ELAS and, in 1945, he was elected to its Central Committee. In July 1951 he founded the United Democratic Left (EDA) party, which he led until 1967, when the colonels' regime persecuted United Democratic Left members.
Passalidis took part in all the Greek legislative elections from 1952 to 1964 as the leader of the United Democratic Left, being among the protagonists of almost all the debates during this period in the Hellenic Parliament.
He was prosecuted and put into in-house arrest by the 1967 junta. This worsened his health and he died on March 15, 1968. He is still remembered fondly among Greek leftists, who still refer to him as "Barbayiannis" (uncle John)