Kelleher's regret at leaving Africa was a significant factor in his becoming a writer. He turned his nostalgia into fiction by writing firstly short stories, and then novels. In 1979 he wrote his first novel, Voices from the River. Further writings on Africa include Africa and After, written in 1983 and republished in 1987 as The Traveller, and Em's Story, written in 1988, which describes a turn-of- the-century journey through the Kalahari desert undertaken by Emma Wilhelm and recorded sixty years later by her granddaughter, Eva.

His son's dissatisfaction with children's books led Kelleher to take a new and very successful direction into children's fiction, subsequently winning three Children's Book Council Awards, a Peace Prize for children's literature and various children's choice awards that are especially important to him. In Forbidden Paths of Thual, his first children's novel , Kelleher drew on his own childhood memories to write in ways that appeal to young children, recreating a world that delighted him as a child. In addition to young adult fiction, Kelleher has written further for adults, including a science fiction novel The Beast of Heaven, about nuclear weapons and the effects of war, which owes much to the Australian landscape. His well-known 1992 novel Micky Darlin' is made up of nineteen linked stories, set in wartime and postwar London and recounts the experiences of an Irish boy and his relationship with his grandparents. Kelleher's Red Heart, published in 2001, focuses on global warming and environmental destruction.

A long-time resident of Sydney, he left there once his children had grown up. Currently, he lives on a country property near Bellingen, NSW, which he shares with his wife, who is a sculptor. Outside of writing, his interests include movies, travel, singing, bush walking, and reading.