A significant number of structural damages observed in reinforced concrete (RC) structures that occurred in regions affected by earthquakes are due to the presence of the infill masonry walls (IMW). Survey of the structural damages caused by earthquakes reported that the infills panels had an important contribution in the seismic response of the buildings, namely through the modification of the natural period of the structure due to the increase of the structural lateral stiffness provided. Different failure mechanisms were observed, in particular the infills’ out-of-plane (OOP) collapse that can have important consequences for the structure and for the human life’s. The infills’ OOP seismic performance depends on different variables such as the wall geometry, existence of openings, boundary conditions, slenderness and existence of previous damage. This manuscript presents an experimental study of ambient vibration tests on IMW with the main goal of characterize the dynamic properties such as the in-plane and OOP frequencies of the panel and the corresponding vibration modes. For this, two different campaigns were carried out: i) in-situ tests were carried out in IMW of three infilled RC structures with aim of capture the OOP frequencies and vibration modes of infill panels with different characteristics and evaluate the influence of each variable; ii) laboratory tests were carried out on a full-scale infilled RC frame in order to evaluate the natural frequencies evolution along the period after construction. Additionally, it was evaluated the effect of the axial load in the adjacent RC columns and also the existence of damage on the IMW natural frequencies and corresponding vibration modes. From the results, it was observed that the openings reduced the OOP frequencies around 20–40%, the axial load increment in the adjacent columns increased the OOP stiffness and consequently the natural frequencies of the panel.