Mohamad al-Amial, the coastal city's municipal council spokesman said on Wednesday that the mass grave was uncovered in an agricultural area of al-Daheir, west of Sirte.
"Seventy-five decomposed bodies were recovered" in the presence of the public prosecutor, Sirte's security force and the Red Crescent, Presstv Reported.
Amial said the find was made "a few days ago" and that the bodies were believed to be of Daesh members , although there was no confirmation.
DNA samples had been taken for analysis.
Daesh exploited the disarray that emerged in Libya following the 2011 overthrow and death of longtime dictator Muammar Gaddafi, taking over a strip of coastal land, including Sirte, in June 2015.
Located on the Mediterranean coast, the city was the main urban center that Daesh managed to seize outside Iraq and Syria.
Forces of Libya's UN-backed unity government, known as GNA, expelled Daesh from the coastal city in December 2016 after eight months of deadly combat. The group has since pulled back to desert outposts.
Since their defeat in Sirte, the militants have kept up the fight from rural areas of western Libya. In April, the Tripoli-based unity government launched a campaign to flush them out.
The terrorist group claimed responsibility for a an attack last month on the Tripoli headquarters of Libya's National Oil Corporation that killed two staffers as well as an assault in May on the country's electoral commission in which 14 people died.
Despite the formation of the internationally-recognized GNA, which emerged from two competing governments in Libya in December 2015, the country has yet to become administratively unified. Military factions allied with renegade General Khalifa Haftar continue to act on their own and without coordination with GNA. There have been reports that Haftar loyalists have been in contact with foreign forces conducting drone operations in Libya as well.
Forces from several Western countries, including the United States, France and Britain, are in Libya in a purported drone war on Daesh.
GNA has denounced the presence of the foreign forces, saying it violates Libya’s sovereignty.