Peter Edward Kassig, 26 years old at the time he was killed, was born and raised in Indianapolis, Indiana. He was the only child of Ed, a school teacher, and Paula Kassig, a nurse.

He attended North Central High School in Indianapolis, graduating in 2006. Kassig then became a U.S. Army Ranger, with an army special operations unit, 1st Battalion, 75th Ranger Regiment. His service including training in Fort Benning, Georgia, and a four-month deployment to Iraq, from June 2006 to September 2007, when he received a medical discharge. Thereafter, he was a student at Hanover College (which he attended from 2007–09) and Butler University (which he attended from spring 2011 to 2012, majoring in political science).

Kassig next worked in Syria and Lebanon as a humanitarian worker. He aided Syrian refugees through Special Emergency Response and Assistance (SERA), a non-governmental organization he founded in the Fall of 2012 to provide refugees in Syria and Lebanon with medical assistance, supplies, clothing, and food. Kassig trained as a medical assistant, provided trauma care to Syrians who were injured, and helped train others to provide medical aid.

On October 1, 2013, as he was on his way to Deir Ezzour in eastern Syria to deliver food and medical supplies to refugees, Kassig was taken captive by ISIL. While in captivity, Kassig – formerly a Methodist – converted to Islam and changed his name to Abdul-Rahman Kassig, sometime between October and December 2013. He was kept in a cell with French journalist Nicolas Henin and British journalist John Cantlie, and beaten regularly.

Kassig was named as the next victim to be beheaded in the video released by ISIL on October 3, 2014, that showed Alan Henning's beheading, and he was shown in the video wearing a Guantanamo Bay-style orange jump suit.

 On October 6, his family sent a video message to the Islamic State and asked for mercy for their son. Kassig's mother later tweeted an entreaty to the leader of the Islamic State over Twitter, asking to communicate with him, and Kassig's parents maintained Facebook and Twitter accounts dedicated to raising awareness of their son’s captivity.

On November 16, 2014, ISIL posted a video showing Jihadi John standing over a severed human head. The beheading itself was not shown in the video. The White House later confirmed the person killed was Kassig. The Telegraph and security expert Will Geddes speculated that Kassig may have defied his captors and refused to provide a beheading video statement.

On 16 November 2014, in the same video which depicted Peter Kassig's death, ISIS also included the beheading of 18 Syrian soldiers in gruesome detail. The BBC pointed out that that this video "revels in gore", and unlike previous videos, this one showed the faces of many of the militants and provided the location as Dabiq in Aleppo Province.