Darryl Robert Cox was born in a U.S. military hospital in Wiesbaden, Germany. His father was Wayne Windell Cox, an NCO in the U.S. Air Force who eventually achieved the rank of Senior Master Sargeant before his retirement from the Air Force in 1972. His mother was Marian Elizabeth Cox (Thomas). Darryl was the third of three boys (his older brothers were Dan and Frank), with another sister and brother (Nancy and Rick) born after him. As an "Air Force brat" he grew up on or around U.S. embassies and military bases from Yugoslavia to Texas to Spain to Wyoming to Louisiana to England for his entire childhood, before his family finally settled in his father's home state of Oklahoma when he was 14.

He first became seriously interested in acting at Del City High School in Oklahoma at age 15, but when he went to the University of Oklahoma he decided to become a naval officer through the OU NROTC program and was commissioned as an Ensign in May of 1977. He served most of his four years on a destroyer, the USS Forrest Sherman, before deciding to resign his commission in May of 1981 at the rank of Lieutenant and return to OU, determined to return to his first love, acting, at the OU School of Drama graduate acting program.

After two years of graduate work at OU he determined that it was time for him to seriously pursue a professional acting career, focusing on film and television. He began by moving to Dallas, Texas in July of 1983 after signing with an agent there. For the next four years he built his career from the ground up, doing mainly commercials and industrial video work before beginning to book principal roles in film and television and becoming eligible to join SAG and AFTRA. Crucial in developing his career was his training at Film Actor's Lab, run by Adam Roarke. In 1987 he married Carolyn Susanne Ingels and joined SAG/AFTRA, moving to Los Angeles.

After three years in Los Angeles, where he not only worked as an actor but as a writer/producer at a promo production company, Davis*Glick Productions, he returned with his wife to Dallas where he signed with the top regional agent at the time, Kim Dawson Agency. He had previously begun teaching Acting for the Camera, first at Film Actor's Lab and then at the American Film Institute in Los Angeles. This continued along with his acting until he received an offer to teach Acting for the Camera at Oklahoma City University, and then at the University of Oklahoma. He returned to Norman, Oklahoma while still retaining the Kim Dawson Agency in Dallas less than three hours away, and in Norman his wife gave birth to a daughter, Alexandria Camille Cox, in 1995.

He has had a career divided between acting and teaching Acting for the Camera (also at the University of Central Oklahoma and other locations, including the Actor Factory in Norman). He has been directed in his many film and television roles by, among others, Francis Ford Coppola, Wes Anderson, Clint Eastwood, Terrence Malick, Paul Verhoeven, Sterlin Harjo, Bille August, Mark Pellington, Lee Isaac Chung, Paul Dano, Tim Blake Nelson, Adam Arkin, and Oliver Stone. Actors he has worked opposite include Owen Wilson, Jeff Bridges, Jake Gyllenhaal, Tim Robbins, Helena Bonham Carter, Hal Holbrook, Mira Sorvino, Jack Warden, Liam Neeson, Steven Yeun, Joan Cusack, Jason Bateman, Patrick Swayze, Zahn McClarnon, Lou Diamond Phillips, Brandon Lee, Peter Coyote, Armand Assante, Michael Chiklis, Scott Bakula, Helen Hunt, Dennis Quaid, Jimmy Smits, Luke Wilson, Martha Plimpton, Aiden Quinn, Bill Paxton, Dennis Franz, Kelly Preston, Terry O'Quinn, Ron Perlman, Halle Bailey, Timothy Dalton, Chuck Norris, Miguel Ferrer, Peter Weller, Marina Sirtis, Sean Young, JoBeth Williams, Kevin Sorbo, Alan Ruck, and Howard Keel.