Their report documents the recruitment and direct participation of children in hostilities, plus the enforced disappearance and extrajudicial killings of prominent women.
The mission established that from late 2019, Turkey facilitated the recruitment of Syrian child mercenaries aged 15 to 18 to fight alongside the Government of National Accord's forces.
The unrest in the north African country has had a dramatic impact on Libyans' economic, social and cultural rights, said the mission.
"All parties to the conflicts, including third states, foreign fighters and mercenaries, have violated international humanitarian law, in particular the principles of proportionality and distinction, and some have also committed war crimes," said Mohamed Auajjar, who chaired the three-person mission.
The mission said it had identified individuals and groups - both Libyan and foreign - who may bear responsibility for the violations, abuses and crimes.
The list will remain confidential until it can be shared with appropriate accountability mechanisms.
Rights situation 'dire'
In June 2020, the Human Rights Council - the UN's top rights body - adopted a resolution calling for a fact-finding mission to be sent to Libya. The move had Tripoli's support.
The experts were charged with investigating alleged violations and abuses of international human rights and humanitarian law in Libya since 2016.
Mr Auajjar was joined by fellow human rights experts Chaloka Beyani and Tracy Robinson.
The experts said more time is needed to probe further allegations of the direct participation of children in hostilities.
Libya has been torn by conflict since the 2011 toppling and killing of dictator Moamer Kadhafi, with rival administrations vying for power.
The mission said civilians had paid a heavy price, notably due to attacks on schools and hospitals, while anti-personnel mines left by mercenaries in residential areas have killed and maimed civilians.
The report said reliable information indicated that people from the Russian private security firm Wagner were operating as mercenaries.
There are "reasonable grounds to believe that Wagner personnel may have committed the war crime of murder" in the shooting of detainees south of Tripoli in September 2019.
The investigation established that hundreds of mostly Russian-made landmines had been planted next to houses in April-May 2020.
Meanwhile, migrants seeking passage across the Mediterranean Sea to Europe are subjected to a litany of abuses in detention centres and at the hands of traffickers, said Dr Beyani.
The report also said that some prisoners are tortured on a daily basis in jails.
"Arbitrary detention in secret prisons and unbearable conditions of detention are widely used by the state and militias," said Ms Robinson.
The mission said there were allegations of "widespread abductions, systematic torture and mass murders" in the town of Tarhuna southeast of Tripoli. Their report included images of several potential mass grave sites.
"The scale of the atrocities in Tarhuna demands far more focused attention," Ms Robinson told a press conference.
Authorities in Libya on Monday announced the discovery of 10 bodies in a new mass grave in Tarhuna, the latest morbid find after years of rule by the notorious Kaniyat militia.
"Two sites were discovered. Four unidentified bodies were extracted from the first and six from the second," the department charged with searching for remains said, adding that it expected to unearth more bodies.
The fact-finders' report will be presented to the Human Rights Council in Geneva on Thursday.
The experts want the council to extend their mandate for a further year.