He's been speaking at the Counter-Daesh Coalition Foreign and Defence Ministers meeting in Washington.
On his first trip to the US in his new job, he said the UK would take a lead in bringing the group to justice.
Speaking in a TV interview he explained the importance of hearing the voices of victims. He said: "There are thousands of them and we need to start setting in train the process of gathering evidence, of getting more witnesses, so that ultimately they can be prosecuted and held to account for their crimes against humanity and that's something that I said today to everybody and got a large measure of support."
His comments were welcomed by Robert Clarke from ADF International - a Christian group which has been calling for the actions of Islamic State to be called genocide.
Speaking on Premier's News Hour he said he was encouraged by the Foreign Secretary's words.
"I think it's exceptionally positive.
"To date we've seen a reluctance from the UK Government to say the right things and do the right things in respect of the millions and millions of Christians and other religious minorities who have been forced from their homes in Iraq."
Clarke said Boris Johnson had championed the use of the word 'genocide' in relation to the events in the Middle East before taking on the role of Foreign Secretary and hoped he would continue to do so.
Meanwhile the UK government has announced plans to give £50m of humanitarian support to the most vulnerable in Iraq who've suffered at the hands of Islamic State.
International Development Secretary Priti Patel said: "The UK is at the forefront of international efforts to tackle Daesh and its poisonous ideology.
"This new support will save lives and alleviate suffering while continuing to underline the UK's commitment to taking its international obligations seriously and leading from the front when dealing with the big challenges of our time, including conflict and instability."
The latest funding brings the UK's total contribution to Iraq since 2014 to £129.5m.
Over 3 million people have been forced to flee their home in Iraq. A further 10 million are in need of humanitarian assistance.
Listen to Rob Clarke speaking to Premier's Hannah Tooley on the News Hour: