He was born in Catania (Sicily), but later he moved to Switzerland with his parents and attended Swiss-German schools until high school. After graduation, he decided to move to the United States to pursue his college education. He spent two years at the University at Albany, The State University of New York, after which he was accepted as an undergraduate at UCLA. The Swiss government's scholarship helped him through five years of UCLA and in 1985 he earned his Master of Fine Arts from the UCLA Film School.

After his MFA, a couple of short films, a screenplay, two video documentaries, and a 16 mm thesis film, he returned to Europe to pursue his Desire to become a film Director. Shortly afterwards, he returned to United States on a work visa to pursue his career in Hollywood as a film Editor. He began as an Editor on Andrei Konchalovski's Shy People. Later, he received an assistant Editor position working with Oliver Stone. However, it was not easy to get the job. Scalia admired Oliver Stone's work, especially Salvador, so he decided he wanted to work with that Director. He got a contact through the sister of one of the assistant editors. Scalia worked on such films as Wall Street (1987) and Talk Radio (1988). He later continued as an associate Editor on Born on the Fourth of July and as an additional Editor on The Doors.

Pietro Scalia worked with Bernardo Bertolucci on Little Buddha (1993) and Stealing Beauty (1996), as well as with Sam Raimi on The Quick and the Dead (1995). He earned two more Academy Award nominations: first in 1997 for Good Will Hunting and second in 2000 for Gladiator, and a second Academy Award for Director Ridley Scott's Black Hawk Down. He also edited G.I. Jane and a pilot episode of a TV series American Gothic in late 1990s.

In the recent years, Scalia edited such movies as Levity (2003) directed by Ed Solomon, a documentary entitled Ashes and Snow, The Great Raid directed by John Dahl, and Memoirs of a Geisha, one of the most publicized movies of 2005, directed by Rob Marshall. Scalia also worked on Hannibal Rising, a movie that tells a story of a teenaged Hannibal and his young sister Mischa Lecter after their parents are killed in World War II. It was directed by Peter Webber and released in 2006. He has a long lasting relationship with Ridley Scott working on movies such as American Gangster in 2007, Body of Lies in 2008 and Robin Hood in 2010. Most recently he worked with Director Ridley Scott on The Martian, released in October, 2015. In May 2017, Scalia replaced Editor Chris Dickens on the upcoming Star Wars film Solo that is due for release in May 2018.