Loach is a long time Corbyn supporter, having joined the Labour leader on anti-austerity protests, and he is among the speakers expected at separate conference being run by the Corbyn-backing Momentum campaign alongside the official Labour event next week.
But he is also a giant of the UK's film industry, with acclaimed movies like Kes and The Wind That Shakes The Barley.
His most recent film - I, Daniel Blake - won the Palme D'Or at the Cannes Film Festival.
Loach's Corbyn documentary follows the Labour leader through his battle against Owen Smith for the leadership of the party, and was launched at two phone bank events backing the Islington North MP last night.
Corbyn described Loach as "one of the greatest directors of our time", while Loach added that he took on the project in a bid to meet party members "inspired" by Corbyn.
"Over two days travelling to events and meetings with Jeremy I documented the sense of optimism and hope that explains why so many are ready to fight for the new settlement that Jeremy Corbyn offers – a vision for our country which in government can implement a shift in wealth and power to ordinary people," Loach said.