Due to a series of unique merits, modular multilevel converters (MMCs) have been developing dramatically in the last decade. However, in some applications, such as offshore power transmission, the MMC modules are subjected to a variety of tough temperature profiles with adverse working conditions, and all of these could lead to thermomechanical fatigues in the components and the joints of the power modules, thereby causing reliability challenge. This paper focuses on the lifetime evaluation of the MMC based on power cycling and thermal cycling, considering the mission profiles in the high-voltage direct current application for offshore wind power. The mission profiles as well as the topology of the MMC are first analyzed in order to better understand the loading characteristics of MMC. Then, the power loss calculation along with the electrothermal simulation is conducted to uncover the thermal loading of MMC. At last, the lifetime of MMC is studied based on rainflow counting and lifetime models for power semiconductors.