Born in the Napa Valley, California, Juan settled in Australia and began teaching at the University of Sydney in 1978 in the Department of Education which is now the Faculty of Education and Social Work. He retired in 2009 but remains the Ashley Montagu Fellow for the Public Understanding of Human Sciences.

Juan has written twelve books including The Odd Body 1, 2 & 3, The Odd Brain, and The Odd Sex, as well as writing columns in The Sydney Morning Herald, The Sun-Herald , The Daily News , The Register , and The National Post . His various books have been translated into numerous languages. The Odd Body published in 1995 appears in 27 languages. The Russian language version of The Essential Odd Body, the combined edition of The Odd Body and The Odd Body 2, was on the best-seller list for 47 weeks in 2008-2009 with 69,000 copies in print. His 12th book, Can Kissing Make You Live Longer? Body & Behaviour Mysteries Explained, was published in February 2010 by HarperCollins Publishers Australia.

As of 2010 he has a regular segment on "Today" on the National Nine TV Network in Australia and consults for and appears in 10 of the 11 episodes of the fifth series of the documentary series "Taboo" on the National Geographic Channel.