Bereket Simon was arrested in January 2019 at his residence in the capital, Addis Ababa, and taken to Bahirdar, capital of the Amhara region - the second largest of Ethiopia’s nine states.
He has been in detention since.
Bereket was found guilty on Tuesday. Some of his supporters had said the charges were politically motivated.
He was a powerful figure in Mr Mele's administration and was often described as the chief ideologue of the ruling coalition, the Ethiopian People’s Revolutionary Democratic Front (EPRDF), which came to power in the early 1990s.
It consisted of the Amhara Democratic Party, the Oromo Democratic Party (ODP), the Tigrayan People's Liberation Front (TPLF) and the Southern Ethiopian People's Democratic Movement.
In 2018, his party Amhara Democratic Party ousted him from executive committee membership.
The EPRDF has since been re-branded and restructured as the Prosperity Party under Prime Minister Abiy Ahmed - and no longer includes the Tigrayan People’s Liberation Front (TPLF).