He has appeared in films such as Milk, Out of the Furnace, Run All Night, A Walk Among the Tombstones, O.G., The Skeleton Twins, and Gone Girl, and starred in the Netflix series Narcos (2015-2017) as DEA agent Steve Murphy and The Sandman (2022-) as Corinthian. In 2012, he also starred in the series Hatfields & McCoys as "Cap" Hatfield. He starred as Donald Pierce in Logan (2017), Quinn McKenna in The Predator (2018), Thomas Lockhart in In the Shadow of the Moon (2019), and Ty Shaw in Vengeance (2022).
Holbrook was discovered while working part-time as a carpenter for Jenny Wiley 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, Versace, Hugo Boss, Bill Blass, Calvin Klein, Moschino, Marc Jacobs and 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 (ISBN 2-914171-20-X).
He released a short series of poems through the website Model-Max.com, accompanied by illustrations by Jamie Strachan.
Holbrook studied acting with the William Esper Studio's two-year Meisner Acting Program and with Shane Ann Younts' two-year voice and speech program, and he received an SCPS Certificate in 16mm film from New York University. He also has worked with acting coach Terry Knickerbocker.
His prior acting work includes a role in the "Allison" music video for Permanent Me 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", at the Rare gallery in Chelsea, New York from April 19 to May 17, 2008.
In 2014, Holbrook appeared as Jeff in David Fincher's psychological thriller Gone Girl.
Holbrook portrayed DEA agent Steve Murphy in the Netflix series Narcos. In 2017, Holbrook played Donald Pierce, the antagonist in Logan, the sequel to The Wolverine. In 2019, Holbrook starred as Thomas Lockhart in the Netflix thriller film, In the Shadow of the Moon.
In January 2021, Holbrook was cast as The Corinthian in the Netflix adaptation of The Sandman.
In May 2021, Holbrook was cast in the untitled fifth Indiana Jones film.