This paper expands on the impact of the thermally induced deformation of the soil on the serviceability mechanical performance (i.e., deformation-related) of energy pile groups. The work is based on the results of a full-scale in-situ test that was performed on a group of energy piles at the Swiss Tech Convention Centre, Lausanne, Switzerland, and on a series of 3-D thermo-mechanical finite element analyses that were carried out to predict the considered experiment. This study proves that the serviceability mechanical performance of energy pile groups crucially depends on the relative thermally induced deformation of the soil to that of the energy piles. The relative thermally induced deformation of the soil to that of the energy piles is governed by (i) the thermal field characterising the energy pile group and (ii) the relative thermal expansion coefficient of soil to pile. Considering these aspects in the analysis and design of energy pile groups is key because they profoundly characterise the deformation of such foundations.