He also performed at the 29th birthday celebration of the New York Pops, which was a celebration called "Journey On", celebrating the work of Stephen Flaherty and Lynn Ahrens. He performed "Wheels of a Dream" from Ragtime with Andrew Rannells, Rebecca Luker, Liz Callaway, Kevin Chamberlin, Jason Danieley, Boyd Gaines, Lewis Grosso, Norm Lewis, Kecia Lewis-Evans, Darlene Love, Marin Mazzie, Tiler Peck, Nikki M. James, Rachel York, and LaChanze, along with Flaherty and Ahrens.

The actor's middle name by birth is d'Arcy. As there was another actor named Brian James in Equity, he uses his full name as his professional name. He graduated from Northwestern University's School of Communication. He received a nomination for the Tony Award for Best Featured Actor in a Musical in 2002 for his portrayal of Sidney Falco in Sweet Smell of Success, co-starring John Lithgow. He received an Obie Award for his performance in the one-man play The Good Thief by Conor McPherson.

Additional Broadway credits include playing Frederick Barrett in Titanic, Lincoln Center's Carousel, and Blood Brothers. Off-Broadway credits include Andrew Lippa's The Wild Party in 2000 opposite Julia Murney and Idina Menzel for which he received a Drama Desk Award nomination, as well as Adam Guettel's Floyd Collins and the Gershwins' Pardon My English. He appeared in Martin McDonagh's The Lieutenant of Inishmore on Broadway, replaced Norbert Leo Butz in Dirty Rotten Scoundrels, and starred in The Apple Tree opposite Kristin Chenoweth. In 2004, he released a Christmas album titled From Christmas Eve to Christmas Morn. He was the Broadway version of Bob Wallace, whose character was originated by Bing Crosby, in White Christmas in 2004. He played the role of Dan Goodman in the new musical Next to Normal Off-Broadway at Second Stage Theatre in 2008. He then starred opposite Daniel Breaker, Sutton Foster and Christopher Sieber as the titular character in Shrek the Musical. The show began previews on Broadway November 8, 2008 and opened on December 14 at The Broadway Theatre after a tryout in Seattle. For this role he won the Outer Critics Circle Award for Outstanding Actor in a Musical and the Drama Desk Award for Outstanding Actor in a Musical. He was also nominated for the Tony Award for Best Leading Actor in a Musical for his portrayal. He departed the cast after one year in the role and was replaced by Ben Crawford.

On July 19, 2010 James performed in front of President Barack Obama and First Lady Michelle Obama at A Broadway Celebration: In Performance at the White House, which also aired on PBS October 20, 2010. He returned to Time Stands Still when the show returned to Broadway. It closed on January 30, 2011.

James was part of the cast of the NBC musical series Smash, playing Frank, who was the husband of Debra Messing's character. NBC officially picked up Smash as a series on May 11, 2011. The program made its series premiere on February 6, 2012. He did not return to the show as a series regular for its second and final season. James starred in Torstein Blixfjord's 2012 short film Bird In A Box.

James starred as Bick in the new musical, Giant, which ran at the Public Theater from October 26 - December 16, 2012. For this role James received a nomination for Best Actor in a Musical at the Drama Desk Awards and Drama League Award for Distinguished Performance.

He starred in the Broadway musical Something Rotten!, which opened in previews at the St. James Theatre on March 23, 2015, and officially on April 22, for which he received his third Tony nomination. James played Matt Carroll in the 2015 film Spotlight. In March 2016, James was cast in the CBS pilot, Superior Donuts, but his role was recast after the show went through some retooling. He reprised his role as King George III in the Broadway production of Hamilton for a limited engagement that began on April 13, 2017 and continued through July 16, 2017.

In 2017, he portrayed Andy Baker in the critically acclaimed Netflix drama series, 13 Reasons Why. James is set to portray Joseph A. Walker in Damien Chazelle's First Man.