IRO is one of the oldest if not the oldest inter-faith organisation in the world. It had its beginnings in the founding of the Inter-Religious Organisation of Singapore and Johor Bahru in 1949. During an inter-faith reception to welcome the renowned Muslim missionary Maulana Abdul Aleem Siddiqui to Singapore in 1949, a group of religious leaders and Sir Malcolm Macdonald (the then British Comissioner General for South East Asia) discussed and agreed with the Maulana to form an inter-faith organisation.

On 18th March 1949, the IRO held its public launch in the Victoria Memorial Hall. A multi-religious capacity crowd of more than 2000 people filled the hall to hear various speakers from different religions affirm their commitment to inter faith endeavours. Sir Malcolm Macdonald described "this meeting is in some ways the most remarkable held in Singapore. Perhaps it is also the most hopeful". At this meeting, Reverend Dr H.B. Amstutz, the first President of the IRO, expressed the belief that "we through this organisation are no longer strangers and enemies but pilgrims on a common road seeking common goals".

In 1961 the organisation was renamed the

Inter-Religious Organisation of Singapore