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.