Born in Rome, Giallini is the son of a housewife and a laborer in a furnace. After attending the Accademia Nazionale di Arte Drammatica Silvio D'Amico, he began acting on stage, working among others with Arnoldo Foà and Angelo Orlando. He made his film debut in 1995, in the Orlando's comedy film L'anno prossimo vado a letto alle dieci. He appeared on several TV-series and was co-protagonist, alongside Valerio Mastandrea, of the Rai Tre sitcom Buttafuori. He is also very active in music videos and short films.
Giallini was married to his wife Loredana for 25 years, and they had two children. He remained a widower in July 2011 after his wife was struck down by cerebral hemorrhage.
In 2012 Giallini won the Nastro d'Argento for best supporting actor thanks to his performance in ACAB - All Cops Are Bastards. The same year he won the Ciak d'oro as personality of the year.