Maroulis was born in Brooklyn, New York to Greek American parents, James Constantine and Constance P. Maroulis (born 1938). Both his maternal and his paternal grandparents immigrated to the United States from Greece in the 1920s. He was brought up in the Greek Orthodoxy and he can speak Greek. His older brother, Athan Maroulis (born 1964), is a record Producer and his sister, Anastacia P. Maroulis (born 1968), is a school principal.

He later sang in high school garage bands, including Lady Rain and Milkbone. Maroulis graduated from Ramapo High School in 1993. He and received a B.F.A. in Music Theater from Boston Conservatory. He was in the cast recording of the 2000 musical, Suburban Dreams, which was produced by Composer Stephen Thompson, Richard Kenner, and Jesse Merz. Some years before appearing on American Idol, Maroulis competed on the dating show Elimidate and was an extra in NBC's Law and Order: SVU as well as performing as a voice-over Artist on the Kids WB series Astro Boy.

In August 2004, acting upon his then-girlfriend's suggestion, Maroulis traveled to Washington DC to audition for Season 4 of the television show American Idol. For the 2005 season, Idol producers changed the age range of contestants from 16-24 to 16-28. Along with Bo Bice (also from Season 4) and Taylor Hicks (winner of Season 5), Maroulis was one of the first contestants to benefit from this change. Maroulis and Bice are also notable for being among the first contestants to compete using the rock idiom.

In the fall of 2005, Maroulis was in negotiations to star in a television sitcom called Brooklyn that was to be loosely based on his upbringing as the youngest of three in a large Greek-American family of New York. Disney's ABC network bought rights to the show, courtesy of Grammnet (of Paramount, owned by Kelsey Grammer). However, later ABC passed on the project before it passed the script stage.

Maroulis joined the cast of the Broadway musical The Wedding Singer in the role of Sammy, in September 2006. Along with the other cast members, he contributed a recording of the Adam Sandler "Hanukkah Song" to the 8th Carols for a Cure CD to help raise funds for the Broadway Cares/Equity Fights AIDS organization. That fall, Maroulis also appeared on various episodes of MTV's Little Talent Show as host.

Maroulis was the recipient of the Hellenic Times Scholarship Fund's Humanitarian Award for Artistic Achievement in 2007. These annual awards recognizing outstanding achievement in the Greek-American community are presented to "individuals who have proven that hard work and dedication lead to achievement, thereby setting a perfect Example for the HTSF scholarship recipients to emulate." That same year, he was also honored with an award from New York City Comptroller william C. Thompson, Jr. at the sixth annual Greek Heritage and Culture Celebration; honoring individuals recognized as Leaders in the Greek Community.

March 2008 saw Maroulis joining the cast of American Idol Extra as cohost, along with fellow Idol alum Gina Glocksen and J.D. Roberto. His next major role occurred that June, when Constantine starred as Judas Iscariot in the Premier Theatre Company's production of Jesus Christ Superstar. Constantine had previously played the role of Jesus in an earlier production when he was much younger.

Maroulis was selected as Broadway.com's "Broadway Star of the Year" for 2009, after receiving over 25,000 votes from Broadway fans and readers of the site.

On December 23, 2010, Maroulis and his then girlfriend, former Rock of Ages chorus girl co-star Angel Reed, welcomed the arrival of their daughter, Malena James Reed-Maroulis. Maroulis and Reed met on the show in 2008. On August 12, 2015, Maroulis was arrested for domestic violence after an altercation with Reed. He was then arrested a second time on August 16, 2015 for violating a restraining order, by sending her an email. Maroulis was promptly released and alleged that he had acted in self-defense. On September 16, 2015, Reed asked that all charges be dropped and the court did so.

Maroulis received the 2011 Spotlight Award from the Elios Society, which honors the achievement of Greek-Americans in the arts and entertainment field.

In 2012, Maroulis became the host of a new series on Fuse.tv.com, Unofficial Idol Forum. Maroulis interviewed Idol contestants as well as interacted with American Idol fans via social media sites such as Twitter and Facebook, soliciting questions and feedback about each week's Idol broadcast.

In 2013, Maroulis received a Drama League Award Nomination for a Distinguished Performance Award for his performance in the title role of Jekyll and Hyde.

On July 23, 2014, it was announced that Maroulis would be returning to Rock of Ages as Drew for a twelve-week engagement at the Helen Hayes Theater beginning August 4, 2014. As of October 21, Maroulis' return was extended to January 18, 2015; the show's final closing date on Broadway.

Maroulis was the first-ever recipient of the Loukoumi "Make a Difference" Award, in January 2015. The award recognized his contributions to the entertainment world and in particular acknowledged his ongoing efforts to support and inspire children.

Maroulis released "Here I Come," on January 29, 2016, and "She's Just Rock'n'Roll" in March. On February 18, 2016, with other American Idol alums, Maroulis returned to the show for its fifteenth season to mentor contestants and participate in Duets Week, where he performed "Bohemian Rhapsody" with Shelbie Zora and performed "My Funny Valentine" with Jenn Blosil. At the season finale, Maroulis returned with other former Idol contestants to celebrate the end of the series finale, where he performed a rock medley with Chris Daughtry, Bo Bice, Caleb Johnson and James Durbin, and a country music medley with Bucky Covington, Scotty McCreery, Ace Young and others.

In spring 2016, Maroulis was cast as Nate in Sofia Alvarez' Friend Art from Second Stage Theatre at the McGinn/Cazale Theater. In March 2017, Maroulis joined the cast of the new musical The Most Beautiful Room in New York in the role of Sergio. The production was directed by Gordon Edelstein with book and lyrics by Adam Gopnik and music by David Shire, with its world premiere in May 2017 at Long Wharf Theatre in New Haven, Connecticut. Maroulis was cast as Judas Iscariot in Jesus Christ Superstar for the June 2017 production at the Muny, directed by Gordon Greenberg. That summer, Maroulis was cast as Che in Evita at North Shore Music Theatre in Beverly, Massachusetts, running from September 26 through October 8, 2017. The new musical Bulldozer: The Ballad of Robert Moses, debuting at the Theatre at St. Clement's in New York City in November 2017, directed by Karen Carpenter, with music, lyrics and book by Peter Galperin, featured Maroulis in the title role as Robert Moses. Reprising the title role, Maroulis was cast in the spring 2018 production of Jekyll and Hyde at the Sandler Center for the Performing Arts in Virginia Beach, Virginia.

Maroulis partnered with Scott Bradlee's band Postmodern Jukebox in early 2018 and performed a number of concerts with them around the United States and Canada. He is slated to headline the “Off the Record Music Festival” in Atlantic City, New Jersey in June 2018.