Thomas Gann was born in Murrish, Ireland and trained in medicine in Middlesex, England.
In 1894 he was appoinrted district medical officer for British Honduras, where he would spend most of the next quarter century. He soon developed a keen interest in the colony's Maya ruins, which up to then had been little documented.
He also traveled into Yucatán, exploring ruins there.
Gann discovered a number of sites, including Lubaantun, Ichpaatun and Tzibanche. He made important early exploration at such sites as Santa Rita, Louisville, Belize, and Coba. At Tulum he documented buildings overlooked by previous explorers, including a rare find of a temple with the Pre-Columbian idol still intact inside.
He wrote several books about his travels and explorations.
Thomas Gann retired as British Honduras's medical officer in 1923.
In his later years Gann was a lecturer in Archeology at the University of Liverpool.