Sarkis was born in Diyana, Erbil Governorate of Iraqi Kurdistan in 1962. He is a Nochiya-Assyrian of the Assyrian Church of the East denomination with family origins from the district of Shamezdin in the Hakkari Province of Turkey. He was first elected into the Kurdistan National Assembly in the first Iraqi Kurdistan elections in 1992, beginning his political career. Sarkis Aghajan Mamendo was also the Minister for Finance and the Economy from 1999 to 2006, and Deputy Prime Minister from 2004 to 2006 in the Arbil & Dohuk administration. Aghajan appears to owe his ascendancy within the Kurdistan Democratic Party power structure to his personal relationship with Nechervan Idris Barzani, the Kurdish prime minister. His supporters say that as young men, the two were forced to flee Iraq with their families in 1975 after the United States withdrew its support of the Barzani clan, and so Nechervan grew up with Sarkis on the Aghajan family estate in Iran.
Sarkis is known, in addition to his political roles, for massive development and aid programs he has begun in which he built or repaired dozens of Churches, roads, schools and Assyrian settlements throughout northern Iraq through the "Higher Committee for Christian Affairs" that he established. He is also a major funder and owner of the new Assyrian TV channel Ishtar TV, which is broadcast in three languages (Syriac, Arabic, and Kurdish).
In August 2006, Sarkis was awarded the "Knight Commander of the order of Saint Gregory the Great" by Pope Benedict XVI as a way to honour his work helping the Assyrian Christian community in Iraq. Aghajan was awarded the title, which is one of the highest and most widely recognized pontifical orders, for his contribution to the Assyrian community and his work for Christians in Iraq.
Many Assyrian non-governmental organizations claim that the Kurdistan Regional Government only appointed Aghajan to a high office in the Kurdish government in order to look tolerant towards Assyrians so that they could justify a possible annexation of Assyrian settlements in the Nineveh plains. It has also been speculated that he gets some of his funding from lobbying efforts in the United States by the Assyrian American congresswoman Anna Eshoo for the purpose of rebuilding Assyrian settlements, although he personally claims that the money is raised by Kurdish political parties instead.