Kiernan was born in Sydney to George Arthur and Leslie Katherine Kiernan. He was educated at The Scots College in Sydney, The Armidale School in northern New South Wales, and the Sydney Technical College, where he trained as a builder.
Kiernan is a passionate yachtsman, sailing competitively for more than 40 years and representing Australia at the Admiral's, Southern Cross, Dunhill, Clipper, Kenwood and Trans Pacific Cup competitions. In 1986/87 Kiernan represented Australia in the BOC Challenge solo around-the-world yacht race. He finished 6th out of a fleet of 25 yachts from 11 nations, setting an Australian record for a solo circumnavigation of the world.
It was during the BOC Challenge that Kiernan saw and was appalled by the amount of rubbish choking the world's oceans. With the support of a committee of friends, he organised a community event – Clean Up Sydney Harbour on Sunday 8 January 1989. 40,000 volunteers turned out to help clean up.
The success of the first Clean Up event in 1989 sparked national interest in the simple but effective community-based idea. In 1990, the first Clean Up Australia Day took place on Sunday, 21 January. Again, the response of Australians was astounding. More than 300,000 Australians volunteered their time to help make a difference to the health of their local environment. Since then more than 7 million people have heeded the call through annual Clean Up Australia Days, Friday Schools Clean Up Days and Business Clean Ups. Clean up Australia's Mission is "To inspire and work with communities to clean up and fix up our Earth". Ian Kiernan met with the United Nations Environment Plan to discuss a global Clean Up. The 'Clean Up the World' event took place in 1993. Over three days (17–19 September), over 30 million people from 80 nations turned out to Clean Up their world in May 2009. Currently 40 million people from 120 countries taking part in the annual Clean Up the World in May 2009.
Kiernan's environmental efforts were originally recognised in 1991 when the Australian Government awarded him the Medal of the Order of Australia (OAM). He was awarded Australian of the Year in 1994. During the ceremony he assisted the Premier of New South Wales John Fahey stopping an assassination attempt on HRH Charles, Prince of Wales. In 1995, he was appointed an Officer (AO) of the Order of Australia.
In 1998, he was the recipient of the UNEP Sasakawa Prize. The prize is awarded every year to individuals with an established track record of achievement and the potential to make outstanding contributions to the protection and management of the environment consistent with UNEP's policies and objectives.
He received the World Citizenship Award from the World Association of Girl Guides and Girl Scouts in 1999.
In 2001, he was awarded the Centenary Medal for "service to the Clean Up Australia Campaign and the Clean up the World Campaign". In 2006 Kiernan received a Lifetime Achievement Award from the National Trust of Australia (NSW). The 2007 Reader's Digest Most Trusted Poll voted Clean Up Australia as the country's most trusted environmental charity and chairman Ian Kiernan AO as the fourth "most trustworthy" Australian. In 2008 the same poll Kiernan was ranked number three.
He is also the recipient of Toastmasters International Communications and Leadership Award, The Berger-Sullivan Tourism Award, the International Banksia Award, and the 1999 Building World Citizenship Award.