The Tripoli native, described by childhood friends as warm and engaging but firm when needed, was thrown into the limelight following a December UN-backed power-sharing deal.
Since arriving in the capital last week Sarraj has been struggling to assert his government's authority, suffering a setback on Wednesday as the head of a rival Tripoli administration refused to cede power.
Sarraj, 56, was born into a wealthy and prominent Libyan family that owned a bookshop and stationary store in downtown Tripoli as well as swathes of land around the capital.
A trained architect, Sarraj, who has jet black hair and a white moustache, had a long career in the public sector and was also a businessman.
He worked for the social security administration, the public works department and served as a commissioner in charge of supervising public projects.
He was well into his 50s when he entered politics, following in the footsteps of his father Mostafa al-Sarraj, a member of parliament and cabinet minister under Libya's deposed King Idriss.
Sarraj was elected to parliament in June 2014 but he and fellow lawmakers fled to the eastern city of Tobruk after a militia alliance overran Tripoli and established a rival administration.
Two years later and after months of arduous negotiations, the United Nations brokered a power-sharing deal between rival Libyan politicians under which Sarraj was designated prime minister.
'CALM AND LEVEL-HEADED'
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The international community pressed reluctant Libyans to endorse the deal, seen as vital to stemming a conflict that has seen the Islamic State group establish a foothold in the country.
"His political stature is still very fragile," said Silvia Colombo, a Libya expert with the Rome-based Institute of International Affairs.
She noted, however, that Sarraj took everyone by surprise when he travelled with cabinet ministers to Tripoli last week from self-exile in Tunis.
"He sent a very very strong message, more than what had been expected of him, by landing in Tripoli. This was an unexpected gesture... from a man who until then had never been a leader," she said.
Sarraj arrived at a naval base in Tripoli last Wednesday amid global hope that his administration will end the chaos that has plagued Libya since Moamer Kadhafi's 2011 overthrow.
Friends of Sarraj describe a charismatic figure who should be able to rally support.
"Fayez is an extremely kind man who is always willing to listen to others. He is patient and respectful but this does not stop him from being firm and speaking his mind," said Salim Ben Hemeda, a childhood friend, fellow architect and neighbour.
Another friend, Rida el-Gontrari, described Sarraj as "calm and level-headed".
Sarraj's father was a member of the nationalist Hizb al-Motamar party and had close ties with Bashir al-Saadawi, one of the key figures of Libya's 1951 independence from Italian rule.
The prime minister-designate is married to an architect and the couple have three daughters.