Kelly has portrayed seven different characters on seven different Law & Order episodes between 2002 and 2011: Court Officer #1 (Law & Order: Criminal Intent episode #115 "Semi Professional" 2002); Det. Finch (Law & Order: Criminal Intent episode #318 "Ill Bred" 2004); Kyle Marsden (Law & Order: episode #1503 "The Brotherhood" 2004); Fireman Charlie Hugo (Law & Order: Criminal Intent episode #604 "Maltese Cross" 2006); State Trooper Lawley (Law & Order: SVU episode #819 "Florida" 2007); Elvis Howell (Law & Order: Criminal Intent episode #910 "Disciple" 2010); and Sergeant Forde (Law & Order: Los Angeles episode #113 "Reseda" 2011).
After playing football at Ithaca College, Kelly transferred to New York University where he earned a BA in English & Writing, with the intention of pursuing a career as a Writer. But, after losing a bet and in spite of a profound fear of public speaking, he entered an acting class. He was so terrified that he gave his first monologue (Brick from Cat on a Hot Tin Roof) to a brick wall. While pursuing his acting career, he also entered the amateur boxing circuit in NYC, winning the Superheavyweight division of the NYC Metros tournament. The following year, he entered the NYC Golden Gloves, winning his first fight by knockout, only to have to subsequently drop out of the competition for work on the film The Devil's Own. A few years later, he also earned an MS from Columbia University. He played Lt. Col. Stephen "Godfather" Ferrando in HBO's Generation Kill. After reading the book upon which the adaptation was based, Kelly embarked on a five-month shoot in Africa, from May to December 2007. He also developed a unique, raspy voice for the role. The Boston Globe praised the "fierce edge" Kelly's performance brought to the series, opining, "Godfather leads with the competitive fury of a high school football coach. His eyes are cold fire, and Kelly, like a few other members of this cast, deserves notice at Emmy time next year." On Memorial Day 2009, he was appointed Honorary Marine by the Marine Corps League (Cpl. Philip A. Reynolds Detachment - 203, Freehold, New Jersey) for his work in Generation Kill.