Born in Thrace in the mid-6th century, Tiberius was appointed to the post of Notarius. There, sometime after 552, he was introduced by the Patriarch Eutychius to the future emperor, Justin II, with whom he became firm friends. Under Justin's patronage, Tiberius was promoted to the position of Comes excubitorum, which he held from approximately 565 through to 574. He was present during Justin's imperial accession on 14 November 565 and also attended his inauguration as consul on 1 January 566.
Justin ceased making payments to the Avars, which had been implemented by his predecessor, Justinian. In 569, he appointed Tiberius to the post of Magister utriusque militiae, with instructions to deal with the Avars and their demands. After a series of negotiations, Tiberius agreed to allow the Avars to settle on Roman territory in the Balkans, in exchange for male hostages taken from various Avar chiefs. Justin, however, rejected the agreement, insisting on taking hostages from the family of the Avar Khan himself. That condition was rejected by the Avars and so Tiberius mobilized for war.