Scalia was born Giacomo Tomaso Tedesco in Brooklyn, New York, of Italian and Irish descent. His father was former Brooklyn Dodger Rocky Tedesco. His parents divorced and when his mother remarried his name was changed to Scalia. In 1969 he graduated from Brentwood High School on Long Island. He was drafted third by the Montreal Expos in 1971 as a pitcher. He was injured and never played in the Major Leagues.

Scalia has been married and divorced twice. His first wife is former model Joan Rankin. His second wife is 1982 Miss Universe Karen Baldwin, with whom he has two daughters, Olivia (b. 1987) and Jacqueline (b. 1990).

Scalia was a regular cast member during the final season of Remington Steele in 1987, after which he joined the cast of Dallas in the role of Nicholas Pearce, love interest to Sue Ellen Ewing (Linda Gray). Scalia's character was killed off at the end of the 1987–88 season when he fell to his death after being pushed from a balcony during a fight with J. R. Ewing (Larry Hagman). He returned to the series finale in a dream sequence in which he was married to Sue Ellen.

In 1992 Scalia was cast as Detective Nico "Nick" Bonetti in the short-lived television series Tequila and Bonetti. Scalia replaced another actor in the role during production of the show's first episode. Eight years later, in 2000, Scalia reprised the role of Bonetti in a revival of the series, which was filmed and aired in Italy.

From 1994 to 1995 he starred in Pointman, a television series on the Prime Time Entertainment Network. He was an investment banker framed and convicted of fraud. When eventually cleared, Constantine "Connie" Harper becomes the owner of a Florida beach resort, Spanish Pete's, and aids people in need with the use of former prison mates and "the list".

Scalia is also known for his role as Chris Stamp on All My Children from 2001 to 2003. He was nominated for a Daytime Emmy Award for "Outstanding Lead Actor" for his AMC role in 2002. In 2006, Scalia starred as President Halstrom in The Genius Club for writer/director Tim Chey. The film is about seven geniuses who must try to solve the world's problems in one night.

In 2007 Scalia co-founded Operation American Spirit with Edra Blixseth. The charity was founded to raise awareness and funding for severely injured troops and their families. In a press conference Scalia stated he started the charity in hopes of raising one hundred million dollars for wounded veterans.

In 2009, while filming The Black Tulip, Scalia spent time visiting servicemen and women. Since then he has been a host or speaker for various events on different military bases. He is currently an ambassador for the Coalition to Salute America's Heroes. The Coalition is a 501c3 charity dedicated to making the lives of wounded veterans returning home to be better prepared for the challenges that lie ahead.

The group organized two 1,500-mile bike rides which took place in 2007 and 2008. The rides took place along the West Coast and were intended to raise both awareness and charitable funds for wounded veterans. The tours made stops to interact with veterans at VA hospitals. The group claimed to have raised $88,937 in 2007 and $14,490 in 2008 according to tax filings, though no charitable recipients were ever revealed. According to a July 3, 2011 report by the New York Post, the IRS revoked the tax-exempt status from the "9-11-01 Lest We Forget" charitable group for failing to file a tax return in its decade of existence. That same year the Operation American Spirit charity was suspended by California authorities because of lapses in paying taxes and fees.

In a 2017 interview on Good Day New York, Scalia talked about his multiple tours to Mosul for humanitarian missions.