Some people would argue that the US is an imperial country because it uses its power to try to influence others around the world. However, this falls far short of what imperialism really is.
The main proof that the US is not an imperial country is that it no longer tries to actually keep any territory outside its borders. It does not use military might to subject and rule any other country or territory. An example of this can be seen in Iraq. Some people claimed that this was an example of US imperialism. But the US did not try to directly rule Iraq past a transitional point. It has now gotten out of the country almost entirely instead of trying to stay and dominate Iraq and its oil supplies.
The US does use its power to try to influence others. However, it no longer conquers and holds territory the way a true imperial power does.