The key concern of the study was the in situ assessment of cracks in historic masonry structures by quantitative IR thermography (QIRT). To better understand the potentials and/or limits of QIRT for that purpose, the non-destructive in situ survey composed of QIRT and ultrasonic testing was conducted on a sixteenth century monument, suffering from structural cracks that had occurred in recent years. Its aim was to discover the thermal behaviour and ultrasonic characteristics of cracks in relation to depth and moisture content. The superficial and deep cracks were found to have different thermal responses to exposed conditions which made them easily distinguishable by QIRT analyses. The results of thermal monitoring during the exposure of heating conditions were promising in giving hints of methods for the depth assessment of deep cracks in masonry. The in situ ultrasonic data taken in indirect transmission mode provided knowledge on the depth of cracks that enhanced the accuracy of the non-destructive in situ survey.