Being at the mercy of someone or something implies a position of vulnerability or powerlessness. However, there are several synonyms for this phrase that can be used to convey a similar sentiment. For instance, some alternatives include being subject to, susceptible to, under the influence of, under the control of, within the power of, and in the grip of. These synonyms can be used interchangeably depending on the context and tone of the sentence. Whether you are writing a formal essay or a casual blog post, using a synonym for "be at mercy of" can help you add variety to your language and communicate your message more effectively.