The evergreen European mistletoe (Viscum album) is the most prominent (hemi-)parasitic plant in Central Europe. It deprives its host of water and nutrient salts by penetration with a modified root organ (haustorium). Even with high weight loads—mistletoes can reach an age of more than 20 years and a diameter of over 2 meters—failure of the mistletoe-host connection has never been described. Instead, it is always the entire host branch, together with the mistletoe, that breaks off under overload. This led to our scientific question: ‘What is the hierarchical structure of this stable, multifunctional connection and how can it be characterized biomechanically?’ X-ray microtomography scans including tissue segmentation showed the development of the mistletoe haustorium from several smaller sinkers in young plants to one major, wedge-shaped sinker in old plants. Anatomical analyses illustrated the (sub-)cellular structure of the connection and revealed both a clear line of separation between the cells of the two species at all developmental stages and a lignification gradient in the mistletoe sinkers that makes the connection very resistant to failure. Intact mistletoe-host samples and slices through the attachment region were biomechanically analyzed under tensile loading to failure. Digital image correlation analysis revealed that local strains of about 30% lead to the initiation of cracks, mostly along the mistletoe-host interface. Multiple pre-failure events were evident in the force-displacement curves, indicating that lateral sinkers fail first and delay the disintegration of the entire haustorium. After failure, the roughness of the fracture area was analyzed by digital microscopy and used for further calculation of various biomechanical variables. Correlation analysis showed that strength of the connection is independent of mistletoe age. Based on our findings of this hierarchically structured, multifunctional plant materials system, we see a great potential for bioinspired joining techniques of technical materials such as fiber-reinforced polymers.