In November 2009, Harrison consecutively struck two Taliban machine gunners south of Musa Qala in Helmand Province in Afghanistan at a range of 2,475 m (2,707 yd) using a L115A3 Long Range Rifle. In a BBC interview, Harrison reported it took about nine shots for him and his spotter to range the target, using a bracketing technique. Then, he reported, his first shot "on target" was a killing shot followed consecutively by a kill shot on a second machine gunner. The bodies were later found by Afghan National Police looking to retrieve the weapon (which had already been removed). The first Taliban was shot in the gut and the other through the side. Later in the day an Apache helicopter hovered over the firing position, using its laser range finder to measure the distance to the machine-gun position, confirming it.

After returning from Afghanistan in 2009 he developed post-traumatic stress disorder (PTSD) and was discharged from the army in 2014. He has stated since that:

"I joined when I was 16 and since all of this has happened I felt abandoned, absolutely abandoned by my regiment. ... I spent 22 years loyal to that regiment, putting my life on the line doing tours, and they just hung me out to dry. My trust in people, the armed forces – it's gone."

Harrison has written The Longest Kill, about his life and career as a sniper.