[3][4] He attended Addison Mizner Elementary School and began to develop his acting skills by appearing in the Little Palm Children's Theatre's productions of Stuart Little, Winnie-the-Pooh, and The Pumpkin King (a theatrical adaptation of The Nightmare Before Christmas), as well as appearing as Alex in the small independent film The Wishing Stone, which was filmed in the Lake Worth, Florida area.

When recounting the decision to cast the 9-year-old Monaghan, director and executive producer Jeff Bleckner explained, "There are some people, when you put them up on film, they sort of pop off the screen at you.

[7] The following year, he began a recurring role as Palmer Noid on the Nickelodeon situation comedy Ned's Declassified School Survival Guide.

Subsequent guest-starring television roles followed, including appearances on Criminal Minds, Numbers, The Mentalist, Monk, Three Rivers, Fringe, The Glades, and Terriers.

[9] On the series, which premiered in January 2011, Monaghan portrays Ian Gallagher, the gay bipolar teenage son of a large dysfunctional Chicago family.

In 2011, he guest-starred as Nick Peyton, a teenage boy accused of murdering his Marine father, on the CBS crime series NCIS.

In 2012, he guest-starred as Eddie Sandow, a 19-year-old who creates a hostage situation after his father is accused of sexual assault, on the NBC crime series Law & Order: Special Victims Unit.

In 2014, Monaghan starred as Adam McCormick, a small-town high school athlete who becomes consumed by the death of one of his classmates in the independent drama Jamie Marks Is Dead.

That same year, he appeared as Mason Ashford, the best friend and romantic interest to Rose in the fantasy adventure film Vampire Academy.

He plays the antagonistic twin brothers Jerome and Jeremiah Valeska, who both serve origin stories for the famous Batman villain Joker.