A geothermal energy pile is a revolutionary piling technique that combines a pile foundation with a ground source heat pump system that not only supports the structure but also provides heating and cooling for buildings and bridges. The thermo-mechanical behavior of long energy piles in soft clay has rarely been investigated, despite their increasing utilization. A long floating energy pile with a length-to-diameter ratio of 66.7 was evaluated on its own and monitored in service of the supported structure in the city of Kunshan, China. With vertical mechanical loads, the experiment involved alternate cooling and heating cycles, allowing for careful analysis and assessment of the pile’s temperature, stress, and displacement. Temperature-induced stress, axial force, and friction resistance of the pile shaft, as well as the change in displacement of the energy pile throughout building, were all studied. The field observations revealed without any surprise that a longer energy pile outperformed a shorter one in terms of heating exchange capacity with a more homogenous temperature distribution along the pile. Following a quasi-linear relationship with the temperature variation, the thermo-induced additional axial force soared with the larger length-diameter ratio of the pile and may even reach four times that of the pile under pure mechanical loads. Important additional settlements were also observed especially in cooling conditions. The shaft frictions along the long bored energy pile were found to have a complicated distribution, which requires further investigations.