The 42-year-old bachelor and social-media savvy economist is known for his open-mindedness and willingness to compromise.
But while he is seen as competent, critics complain that his success in a string of posts held so far has been only middling at best.
"He will certainly be a milder, less offensive, and less arrogant prime minister than Robert Fico was," Bratislava-based political analyst Pavol Babos told AFP.
Fico offered his resignation on Wednesday in face of a public backlash over the murder of an investigative journalist.
President Andrej Kiska accepted the decision on Thursday, tipping Pellegrini as Fico's successor.
Described by Fico as a "good friend", "Pelle", as he is nicknamed, has long been perceived as "crown prince" within the Smer-SD party, where he has been deputy leader since 2014.
Slovak-born, but with Italian roots, he has held a number of different positions over the years, from speaker of parliament, education minister, state secretary for finance, deputy prime minister for investments to, most recently, acting culture minister.
But he has yet to prove himself as a real man of action, critics say.
"He had no scandals and no major results in these positions," Babos said.
A close associate who wished to remain anonymous told AFP that Pellegrini is "understanding, friendly and acts as if he were progressive. But when it comes to real action, that's where the story ends."
He told a reporter that he was "deeply affected" by last month's murder of investigative journalist Jan Kuciak, who had been probing alleged high-level political corruption.
The killing and Kuciak's posthumous investigative report sparked a wave of anti-government sentiment and plunged the country into political crisis.
Pilot-in-training
Pellegrini was born on October 6, 1975 in the central city of Banska Bystrica to a car mechanic father and a teacher mother.
According to his mother, Pellegrini loved cars and music, danced in an ensemble and played the accordion.
He studied finance, banking and investment in his hometown and in the eastern city of Kosice, before working as an economist and later as an aide to a Smer-SD lawmaker.
Pellegrini became an MP for the party in 2006 at the age of 29.
Deputy premier since 2016, he notably helped convince Jaguar Land Rover to invest 1.4 billion euros ($1.7 billion) to build a plant in Slovakia -- due to open this year -- thereby beating out competition from neighbouring Poland.
Pellegrini speaks Russian, German and English in addition to his native Slovak.
And he is currently studying for his pilot's licence.
"There are three things I like: modern technology, art and movement," he said once and even hoverboarded to a government meeting one time.
Pellegrini is also very active on social media.
When he uploaded a photo of himself holding a Bengal tiger cub last year, one user commented: "Both beasts look good."
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