In a statement, his family asked the public to refrain from sharing pictures or video of his death and instead would like him to be "remembered for his important work and the love he shared with friends and family".
Born in Indiana, he was deployed to Iraq as a US army ranger in 2007, before going on to set up his own charity that helped refugees from the Syrian civil war.
Special Emergency Response and Assistance (SERA) was set up by Kassig in 2012 to "provide acute logistical support and assistance in areas too difficult for other humanitarian organisations to effectively operate."
The group supplied essential equipment such as blankets, stoves, and fuel to refugees from Syria.
It's about showing people that we care, that someone is looking out for those who might be overlooked or who have slipped through the cracks in the system for whatever reason.
When he first returned to the Middle East after being discharged from the army on medical grounds and spending time at college in the States, Kassig volunteered in a hospital and a refugee camp in Lebanon.
I divide my time between my personal volunteer efforts, my organisations relief operations, which include the distribution of aid materials such as medical equipment and children’s clothing, as well as food and cooking materials in both Lebanon and Syria.
There are a lot of other wonderful organisations out there but we feel that by working directly with the people who are in need at a grassroots level allows for us to establish an invaluable personal relationship.