Vincentas Sladkevičius was born in Žasliai, Kaišiadorys, to Mykolas Sladkevičius and his wife Uršule Kavaliauskaite. He was the youngest of five children, his siblings being named Ona, Emilija, Jonas and Marija. After studying at the Kaunas Priest Seminary and TheologicalFaculty in Kaunas, Sladkevičius was ordained to the priesthood on 25 March 1944. He then did pastoral work in Kaišiadorys until 1959, including serving as a professor and the prefect of studies and discipline at the Kaunas seminary.