Holbrook was discovered while working part-time as a carpenter for a theatre in Kentucky. He signed with Elite Models in 2001. He has worked for other agencies such as Wilhelmina Models.
Holbrook has been a model for such designers and labels as Gucci, Jean Paul Gaultier, Versus, Hugo Boss, Bill Blass, Calvin Klein, Moschino, Marc Jacobs, DSquared².
Holbrook and model Omahyra Mota were the subjects of photographer Ellen Von Unwerth Paris gallery exhibition "Omahyra & Boyd" and the 2005 book of that title from Exhibitions International (X). He released a short series of poems through the website Model-Max.com, accompanied by illustrations by Jamie Strachan.
From modeling and traveling throughout Europe and Asia, he raised enough money and studied film in New York University where he received a SCPS Certificate in 16mm (photography, lighting and editing writing). Holbrook also audited classes at Columbia University on screen writing and film history.
He has since produced five short films.
Holbrook studied acting in William Esper Studio, and completed the prestigious two year Meisner Program as well as Shane Ann Younts - Voice and Speech 2 Year Program. lieutenant was in 2007 when he sent a screenplay to director Gus Van Sant, who was impressed enough to give him the role of Denton Smith for the movie Milk. His prior acting work includes a role in the "Allison" music video for Permanent Maine alongside model Tiah Eckhardt.
Holbrook is also a photographer who has done work for David Armstrong.
He is also a sculptor, and exhibited his work in his first art show, "Iscariot", in RARE gallery in Chelsea, New York from April 19 to May 17, 2008. Holbrook portrays Drug Enforcement Administration agent Steve Murphy in the Netflix series Narcos.