Yost graduated from the University of Michigan with a film and video degree. With no clear sense of which direction to take his career in, he got into advertising in the Detroit area, producing TV commercials. He later interned, in 2002, in Marvel Comics' west coast office. His spec film scripts got attention from Marvel executives who hired Yost to write episodes of the TV series X-Men: Evolution. In 2003, Yost and Kyle co-wrote the episodes that introduced mutant character X-23, female clone of the popular X-Men character Wolverine. Marvel executives were impressed with X-23’s reception on TV, and asked Yost and Kyle to adapt the character into comics, first by writing a six-issue mini-series titled X-23: Innocence Lost, and then by taking over writing chores (as of issue #20) on the New X-Men title, bringing X-23 in as a regular character. Yost and Kyle also co-wrote a second X-23 six-issue miniseries, titled X-23: Target X as well as X-Force. Yost also wrote Ender's Game: Battle School, the first in a series of adaptations of Orson Scott Card's Ender's Game by Marvel.
In 2012 Yost moved to work on Marvel's Spider-Man family of comics, revitalizing the character Kaine from the 1990s storyline, the Clone Saga, in the ongoing series Scarlet Spider. He also took over writing duties of the Spider-Man team-up book, Avenging Spider-Man, which was relaunched as Superior Spider-Man Team-Up in 2013.
Outside of Marvel, Yost has written several other comics including Red Robin from DC Comics and his own creator owned series Killer of Demons from Image Comics.
Yost has also written for such animated shows as Teenage Mutant Ninja Turtles and The Batman and was the story editor and head writer on the Fantastic Four animated series that aired on Cartoon Network in 2006 as well as Iron Man: Armored Adventures. He was the head writer of Marvel Animation's The Avengers: Earth's Mightiest Heroes, which aired on Disney XD from 2010-2012, and has recently worked on Lucasfilm's Star Wars: Rebels on Disney XD.
Yost worked in the Marvel Feature Film Writers Program from 2010-2012 before embarking on his feature film screenwriting career. He was one of the writers of Marvel's feature film Thor: The Dark World (2013),[4] and then wrote Mattel's film Max Steel, before returning to Marvel to work on Thor: Ragnarok, and Sony's Masters of the Universe feature film adaptation.
On June 6, 2017, it was announced that Yost would write an American live-action adaptation of Cowboy Bebop for television with Tomorrow Studios, a partnership between Marty Adelstein and Sunrise Inc., which also produced the original anime. Additionally, he worked on The Mandalorian for Lucasfilm and Disney+.
