Garber was born in London, Ontario, and is of Russian-Jewish descent. His parents were father Joseph "Joe" Garber (died 1995), and mother, Hope Garber (née Bessie Hope Wolf; died 2005), an actress, singer, and the host of At Home with Hope Garber. He has a brother, Nathan, and a sister, Alisa.
Garber began acting at the age of nine, and studied at the University of Toronto's Hart House at age 16.
When Garber was twelve, he was diagnosed with type 1 diabetes. He attended Ryerson Elementary School. He also was enrolled in the children's program of the Grand Theatre; and, at age 16, he was accepted at a six-week summer theatre training program at the University of Toronto taught by Robert Gill.
In 1967, after a period working as a solo folk singer, he formed a folk group called The Sugar Shoppe with Peter Mann, Laurie Hood and Lee Harris. The group enjoyed moderate success, breaking into the Canadian Top 40 with a version of Bobby Gimby's song "Canada" (1967). Three other Sugar Shoppe songs made the lower reaches of the Canadian Top 100 in 1967 and '68, and the band even performed on The Ed Sullivan Show and The Tonight Show Starring Johnny Carson before breaking up.
He played "Jesus" in Toronto's 1972 production of Godspell. In 1985 he appeared in a supporting role in Noises Off at the Aldwych Theatre on London's West End.
He appeared on Broadway in the original productions of Deathtrap, Sweeney Todd and Noises Off, and in the original Off Broadway cast of Assassins, as well as the 1990s revival of Damn Yankees. He has been nominated for four Tony Awards and opened the Tony Awards program in 1994 (the year he was nominated for the Tony Award for Damn Yankees). In 1998, he co-starred on Broadway in the Tony Award–winning play Art with Alan Alda and Alfred Molina. In 2005, he played the role of Frederic in the Los Angeles Opera production of A Little Night Music. He played "Ben" in a critically praised Encores! staged concert production of Follies (2007), opposite Donna Murphy. In mid-2007, he played Garry Essendine in a production of Noël Coward's Present Laughter at Boston's Huntington Theatre. He reprised the role on Broadway in the Roundabout Theatre production, which opened in January 2010.
Garber is set to next appear on Broadway as Horace Vandergelder in the Tony-winning, Scott Rudin-produced revival of Hello, Dolly! at the Shubert Theatre. Replacing David Hyde Pierce and starring opposite Bernadette Peters, Garber will begin performances on January 20, 2018 prior to an official opening night on February 22, 2018.
His film work includes James Cameron's Titanic (1997), in which he essayed a Mid-Ulster accent to play the shipbuilder Thomas Andrews. Other movie appearances include Godspell (1973) as Jesus (the part he originally played in the 1972 Canadian stage production), Sleepless in Seattle (1993), Annie (1999), Legally Blonde (2001), and Tuck Everlasting (2002). In 2009, he took on the role of the DC Comics supervillain Sinestro in the direct-to-video animated film Green Lantern: First Flight. The same year, Garber played a Klingon interrogator in J. J. Abrams' Star Trek film; however, his scenes were deleted from the finished film.
In 2010, Garber had an uncredited cameo in The Town as a bank manager. The film was directed by Alias co-star Jennifer Garner's husband, Ben Affleck. Also Garber had a credited role in the Ice Quake at the end of 2010. In late 2012, he appeared in Affleck's film Argo, about the Iran hostage crisis; Garber portrayed Canadian Ambassador to Iran Kenneth D. Taylor. He also co-starred in 2014 Samuel L. Jackson and Felicity Huffman in the Finnish-British Action Thriller Big Game.
On television, he has had roles on American and Canadian shows. Garber's first leading role on television show was in CBS's 1985 summer series I Had Three Wives. He had a recurring guest role on CTV's E.N.G. (1991–93). He portrayed Jack Bristow on ABC's show, Alias, earning three Emmy nominations. He next starred on the television series Justice (2006) on Fox and ABC's Eli Stone. He appeared as Olivier Roth in four episodes of the Canadian science drama ReGenesis. He appeared in Episode 3 of the Fox series Glee as Will's father. He currently guest stars as Dr. Martin Stein on The Flash and appears as a regular on Legends of Tomorrow. He appeared once on Frasier as Frasier Crane's butler.
In December 2010, for their annual birthday celebration to "The Master", The Noël Coward Society invited Garber as the guest celebrity to lay flowers in front of Coward's statue at New York's Gershwin Theatre, thereby commemorating what would have been Coward's 111th birthday.
In 2013, he was inducted into Canada's Walk of Fame.
Garber prefers to keep his personal life private and has largely stayed out of the tabloids. He referred publicly to his homosexuality in 2012. In 2013, he said "I don't really talk about it, but everybody knows." Garber has been in a relationship with Canadian artist and model Rainer Andreesen since 2000. On October 10, 2015, Andreesen announced on his Instagram page that he and Garber were married in Canada.
Source: PlaybillThe Shadow Box – 1977
Tartuffe – 1977
Deathtrap – 1978 (Tony Nomination, Featured Actor in a Play)
Sweeney Todd – 1979 – as Anthony Hope
They're Playing Our Song – 1981
Little Me – 1982 (Tony Nomination, Actor in a Musical)
Noises Off – 1983
You Never Can Tell – 1986
The Devil's Disciple – 1988
Lend Me a Tenor – 1989 (Tony Nomination, Actor in a Play)
Two Shakespearean Actors – 1992
Damn Yankees – 1994 (Tony Nomination, Actor in a Musical)
Arcadia – 1995
'Art' – 1998
Present Laughter – 2010
Hello, Dolly! – 2018
Godspell – 1972
Ghosts – 1973
Joe's Opera − 1975
Cracks – 1976
Wenceslas Square – 1988
Love Letters – 1989
Assassins – 1990 as John Wilkes Booth