Even before he was named, opponents were suggesting the silver-haired foreign minister had been hand-picked by Renzi as a stand-in leader who is dull enough not to pose any threat to the outgoing premier's comeback hopes.
Renzi, who resigned last week after a crushing referendum defeat, remains leader of his Democratic Party (PD) and has made it clear he plans to fight the next election as its candidate to head a new government.
Aged 62, Gentiloni is a former student radical who comes from a well-to-do Roman family with aristocratic roots.
He has long been associated with the Green wing of Italy's left but has steadily moved towards the centre ground over the course of a long, unspectacular career.
He was made foreign minister in October 2014, having been plucked from obscurity to replace Federica Mogherini following her appointment as the European Union's foreign policy chief.
It was an appointment that raised eyebrows at the time.
The man chosen by Renzi was virtually unknown to the public. He had no experience in foreign affairs and his ministerial career had been limited to a stint as communications minister in Romano Prodi's 2006-08 government.
His bid, in 2013, to become the mayor of Rome ended with him finishing third in a field of three in the left's primaries.
Political analysts concluded Gentiloni was being rewarded for his loyalty to Renzi.
Turin daily La Stampa described him as “A Renzi-ite even before Renzi himself,”, noting how Gentiloni had often provided the outgoing premier with valuable intellectual cover in internal PD battles over the party's direction.
Both men were members of La Margherita (The Daisy), a short-lived centre-left party, in the early 2000s and both were close to and influenced by Francesco Rutelli, a veteran Green and former mayor of Rome.
Gentiloni is married to an architect. The couple have no children and he is said to spend his spare time readying newspapers and books and pursuing his passion for fine wine.
He plays tennis and is an opera buff but friends say he has had little time for leisure activity of late as a workaholic who likes to read his ministerial briefs in full.
Frequently described as a former journalist, Gentiloni does have a press card. But his reporting and editing experience was nearly all for political publications. Like Renzi, he has been a full-time activist or an elected official for most of his working life.
A descendant of a famous Italian count, he has the right to call himself a noble and, according to media reports, lives in a Rome condominium building occupied mainly by his relatives.
Close to US
As a political science student in the mid-1970s, Gentiloni was involved with fringe far-left groups, including one which described itself as being inspired by Maoism.
Like many of his generation, the flirtation with revolutionary politics was short-lived. He is now considered as being in the mainstream of the broad church PD.
As foreign minister he forged a close working relationship with US Secretary of State John Kerry, helped by the Renzi government allowing US airbases in Italy to be used for airstrikes on Islamic State targets in Italy.
After spells as editor of Green magazine “The New Ecology” and as spokesman for Rutelli during his time as mayor of Rome, Gentiloni was elected as a PD deputy in 2001.
He continued to harbour ambitions of becoming mayor of Rome. But after his failure to win the candidacy in 2013 a quiet retirement appeared to be beckoning before he got the call from Renzi two years ago.
He is widely seen as having done a good job as foreign minister as Italy has played an unusually proactive role on the world stage, notably in relation to Libya, and in building bridges with Iran after the lifting of international sanctions.
By Angus MacKinnon