Mohammed Abdallah, 26, from Manchester, was found guilty of membership of IS at the Old Bailey on Thursday and was sentenced on Friday after he was identified in official IS documents obtained by Sky News.
He was also found guilty of possessing an AK-47 assault rifle and receiving £2,000 to fund terrorism.
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Abdallah travelled to Syria with three friends in July 2014 and his wheelchair-bound brother Abdalraouf, 24, stayed in the UK and directed them from the family home in Moss Side, south Manchester, after being shot and paralysed while fighting in the conflict.
He spent two years fighting for the terror group in the Middle East before returning to the UK in September 2016 and being arrested at Heathrow Airport.
The Old Bailey heard that Abdallah was known to the authorities after Sky News obtained thousands of documents containing names, addresses and contact details of registered IS fighters.
The file showed that the former supermarket worker was listed as a "fighter" and specialised as a "sniper".
Abdallah was good friends with Manchester bomber Salman Abedi and they lived just streets away.
They visited the Al-Rahman Islamic Centre in Moss Side together and members told Sky News they were "horrified and sickened" by how their former members turned out.
In mitigation, Rajiv Menon QC said there was no evidence Abdallah was "on a mission" in the two years between leaving Syria after four weeks and the time his involvement with IS emerged, but he has been put behind bars for 10 years with five years on extended licence.