O'Brien sa narodil v New Yorku ako syn Lisy (rodenej Rhodes), bývalej herečky, ktorá tiež prevádzkovala hereckú školu a fotografa Patrika.<ref>{{Cite web |url=http://www.skidmore.edu/scope/fall2009/classnotes/1986.htm |title=Skidmore Scope Fall 2009 |last=Stuzin |first=Anne Chorske |date= |website=Skidmore Scope |publisher= |accessdate=March 23, 2014}}</ref><ref name="Real">{{cite web|url=http://www.teentelevision.com/2011/06/27/teen-wolfs-dylan-o-brien-in-real-life-i-m-the-were|title=Teen Wolf's Dylan O'Brien "In real life, I'm the werewolf."|last=Barker |first=Lynn |website=Teen Television |date=July 27, 2011 |accessdate=July 27, 2011}}</ref>Vyrastal v SptingfieldeSpringfielde v New Jersey, až do veku dvanástich rokov, kým sa so svojou rodinou presťahoval do Hermosa Beach v Kalifornii.<ref>{{Cite web |url=http://www.listal.com/dylan-o%27brien_ii |title=Dylan O'Brien |last= |first= |date= |website=Listal |publisher= |accessdate=September 4, 2013}}</ref><ref name="NewNowNext">{{cite web|title=Dylan O'Brien|url=http://www.newnownext.com/personality/dylan-obrien/|work=NewNowNext|publisher=Viacom|accessdate=October 17, 2013}}</ref><ref>{{Cite web |url=http://www.popentertainment.com/dylanobrien.htm |title=Dylan O'Brien interview about 'The First Time' and 'Teen Wolf.' |last=Jacobs |first=Jay S. |date=October 19, 2012 |website=PopEntertainment.com |publisher= |accessdate=September 4, 2013}}</ref>Jeho otec je írskeho pôvodu a matka má [[Taliansko|talianskych]], [[Anglicko|anglických]] a [[Španielsko|španielskych]] predkov.<ref name="r1">{{cite web|url=http://www.asiaone.com/News/Latest%2BNews/Showbiz/Story/A1Story20120601-349879.html|title=Sidekick supreme|last=Johnson|first=Jason|website=[[AsiaOne]]|date=June 4, 2012|accessdate=June 4, 2012}}</ref> Skôr ako sa dostal k herectvu, chcel chodiť na filmovú školu a byť kinematografiám, ako jeho otec, ale nakoniec sa rozhodol pre herectvo.<ref>{{cite web|url=http://www.asiaone.com/News/Latest%2BNews/Showbiz/Story/A1Story20120601-349879.html|title=Sidekick supreme|last=Johnson|first=Jason|website=[[AsiaOne]]|date=June 4, 2012 |accessdate=June 4, 2012}}</ref> Dylan okrem herectva hrá aj v kapele Slow Kids At Play, v ktorej je bubeníkom a je veľkým fanúšikom New York Mets.
