[4] In 2008, Haynes continued modeling in campaigns such as Verizon and also appeared in numerous magazine editorials such as Teen Vogue and Arena.

In addition to guest roles, in April 2007, Haynes acted in My Chemical Romance's music video for their single "I Don't Love You", directed by Marc Webb.

[8] Later that year, Haynes auditioned for the role of Edward Cullen in the film version of Stephenie Meyer's young adult novel Twilight, but Robert Pattinson was cast instead.

The show is very racy with lots of nudity, sex, drugs and real-life experiences that will shock audiences," Haynes said in an interview.

In March 2010, filming began for the ABC television series The Gates, in which Haynes portrays Brett Crezski, a jock who begins to turn into a werewolf.

Haynes landed the role of Jackson Whittemore in the 2011 MTV series Teen Wolf,[14] based on the 1985 film of the same name.

In October 2012, Haynes revealed he was leaving the show after two seasons, stating "I'm sad that this chapter has ended, but excited for a new one to begin.

[21] Haynes began foraying further into the music industry that same year by covering a song, "19 You+Me", with New Heights, before releasing a single with Travis-Atreo called "Baby It's Christmas".

[27] Haynes featured in the 2018 film Bigger as Jack LaLanne, reuniting with Teen Wolf co-star Tyler Hoechlin.

[29] Later that year, Haynes rejoined Arrow, reprising his role as Roy Harper in the sixth season of the show in a guest capacity.

[31] In August 2019, he confirmed he would not return as a regular for Arrow's eighth and final season, but added that Roy is "never gone for too long".

[37] Not yet ready to speak openly about his sexuality, the renewed attention led Haynes to seek treatment for anxiety for a period of three months.

[48] In March 2019, Haynes, having achieved sobriety, spoke publicly about his experiences with alcoholism and addiction, stating: "In ten years, there were maybe 25 days I didn't drink.