Two months later the party is in crisis, its credibility on the line because the beleaguered new mayor, Virginia Raggi, has still not finalized a team to run the city she vowed to start cleaning up from day one.
Fed up with chaos and corruption in their public services, Romans swept Raggi into power to chants of "Onesta, Onesta" (Honesty, Honesty), Five Star's rallying cry.
After a week of turmoil, Raggi, 37, now stands accused of being anything but honest. She has scored a string of political own goals and, despite hopes to the contrary, has failed to tame what many regard as an ungovernable city.
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Her woes started last month when rubbish began to pile up on the streets of Rome's neglected outlying suburbs, providing a smelly reminder of endemic problems at the city's corruption-tainted refuse collection agency AMA.
Simultaneously, Raggi came under fire for putting a former AMA insider, Paola Muraro, in charge of cleaning up the agency.
'I'm not giving up'
Worse was to follow. Muraro, it quickly emerged, is under criminal investigation linked to her time as an AMA consultant and the mayor knew that.
Raggi told a parliamentary hearing this week that she had informed her own party hierarchy but had not felt compelled to make the information public. Political opponents and much of the media smelt blood: was the squeaky clean party of transparency exposing itself as just like all the others?
Raggi maintains she did not deliberately mislead the public. "I'm not giving up," she vowed.
But her handling of the issue has angered M5S activists and increased tensions between Raggi and Grillo. The pair are reportedly no longer on speaking terms as the mayor resists the party founder's desire to keep her on a tight rein.
Luigi Di Maio, the Grillo protege seen as the movement's likely candidate for prime minister, has also been damaged, having been forced into an embarrassing confession that he had not grasped the severity of the allegations against Muraro. "I made a mistake," he told a rally of party faithful.
Hit in the polls
Raggi's cause was not helped by her decision to appoint a chief of staff on a reported salary of 193,000 euros, twice that of his predecessor.
Despite the generous stipend, he resigned last week, along with the city finance chief and three senior staff at AMA and ATAC, the company that runs the city's much-maligned public transport. A replacement finance chief had not even started work before he too was forced to step aside because of his own legal problems.
Analysts say the fiasco in Rome is bound to damage the national standing of a party that had overtaken the ruling Democratic Party in the opinion polls with around 30 percent of voters pledging to back them in national elections.
"M5S has blundered every step of the way and it is paying the price for thinking practically anyone could run Rome: (Raggi's) three years' experience as a city councillor were never going to be enough," said Gianfranco Pasquino, a professor of political science at the John Hopkins School in Bologna.
"What has happened will have an effect on public opinion and some people who were planning to vote M5S could change their minds," Pasquino told AFP.
Pollster Antonio Noto said the fiasco in Rome had affected the movement's national standing but that it was too early to say if a hit of around three percentage points would be long-lasting.
"For the first time since the movement was created, half its supporters are very critical of how this affair has been handled - that is something new," Noto, of IPR Marketing, told daily Il Messaggero.
"But that does not necessarily mean voters will abandon them, they'll see how the situation develops."