Mientus grew up in Pennsylvania. One of his first large parts was playing Tevye in his high school's performance of Fiddler on the Roof. He studied at Act One Theatre School.
Before starting his career in theatre, Mientus was a fan of the musical Spring Awakening. After seeing the show at the Atlantic Theatre Company, he created a Facebook group for his friends and himself to discuss the show on. However, Mientus began to notice strangers (fellow fans) were joining the group, and soon one of the producers from Spring Awakening contacted Mientus and asked him to make his group the official show Facebook page, which Mientus could moderate. He agreed, and was later recommended by the same producer to audition for the touring company of Spring Awakening. He left the University of Michigan after his junior year to be in the show, and never returned to graduate.
Mientus has toured with the first national touring company of Spring Awakening as Hanschen and in the 2010-11 international tour of Academy and appeared in the 2012 Off-Broadway revival of Carrie: The Musical.
In 2013, Mientus was cast in season two of the musical drama television series Smash as series regular Kyle Bishop. Following the cancellation of Smash, Mientus and co-stars Jeremy Jordan and Krysta Rodriguez joined the cast of Hit List, the real-world staging of the fictional rock musical created for season two of Smash. The show ran for three performances on December 8—9 at 54 Below.
Mientus made his Broadway debut in the 2014 revival of Les Misérables as Marius.
In 2014, Mientus appeared in several episodes of the ABC Family series Chasing Life. That same year, he was cast in a recurring role on the CW series The Flash as the Pied Piper. Mientus made history in his role as Pied Piper by playing the first openly gay supervillain ever.
In February 2015, Mientus was cast as journalist Brett Craig in Parade, for a one-night-only concert presentation at the Lincoln Center's Avery Fisher Hall. After finishing his run of Les Misérables in early 2015 and being succeeded by Chris McCarrell, Mientus moved to Los Angeles with fiancé Michael Arden, where he appeared in the show Bent at the Mark Taper Forum. He also reprised his role as Hanschen in the Deaf West production of Spring Awakening, directed by Arden. The production then transferred to Broadway, and closed on January 24, 2016. Mientus is currently working on his own musical, Manhattan Kids, with the band Teen Commandments. He performed at the club 54 Below in the Hell's Kitchen neighborhood of Midtown Manhattan with the band in August 2016.
Mientus is openly bisexual. He married fellow Broadway actor (and director) Michael Arden on August 18, 2016, at Babington House, Somerset, England. He and Arden planned proposals to each other the same day (June 23, 2014), without the other knowing. They have been together since November 14, 2010. The two first met during the opening night party of the Broadway show The Times They Are-a-Changin', in which Arden was a performer.
He has a rare neurological disorder called HNPP (Hereditary Neuropathy with Pressure Palsies). During his freshman ballet class at the University of Michigan, his foot went numb and he had to pull out of the class for the remainder of the year. He was then put in a stage tech class, where he met fellow actor Darren Criss.