Hillen was raised in Newry, County Down, Northern Ireland. He studied at the Belfast College of Art. In 1982, he traveled to London to continue his studies at the London College of Printing, and then at the Slade School of Fine Art.
Hillen traveled back and forth between Ireland and England over the course of several years. He photographed scenes related to the conflict in Northern Ireland. He later incorporated these photographs into photomontage work. Eventually, he began to take photographs with the photomontage as his priority.
Many of these photomontages have lengthy, comical titles such as "Sr. Faustina Appears In London Newry, Miraculously Preventing The Illegal Photography Of Members Of The Security Forces...". His works on Northern Ireland were published, appearing on the covers of Creative Camera and the Royal Photographic Journal. Taking photos of the Security Forces was, and is, technically illegal. Hillen often pretended to be part of the press or "just an innocent art student".
In 1993, Hillen returned to Ireland. Between 1994 and 1997, he created a new body of work entitled Irelantis. The Irelantis series is a collection of scalpel-and-glue collages. They are a bizarre hybrid of the everyday postcard visuals, mixed with a fantastical other world. Most of these highly decorative collages, showing landmarks from around Ireland, are no bigger than a postcard.
These works are partly influenced by the famous picture postcards of John Hinde. Hillen adapts these nostalgic images, juxtaposing the familiar touristic shots to create an altogether different and complex image.
The Irelantis series celebrates the imagery of Ireland. Hillen illustrates places such as Newgrange, The Cliffs of Moher, Trinity College and O'Connell Street.
In addition to John Hinde, Hillen has been influenced by many other artists, including the great photomontage artist John Heartfield. This influence can be seen especially in Hillen's more political montages of the Troubles in Northern Ireland.
His later activities include photo series such as the ongoing Untitled Broken Umbrella Project, which consists of hundreds of photos of broken umbrellas around Dublin.
In 2007 Hillen won, with landscape architect Desmond Fitzgerald, the design competition for an Omagh Bomb Omagh bomb Memorial.
Hillen now lives and works in Dublin.