As Emperor of France, Louis Napoleon reigned from 1852 to 1870. During this time he was rather aggressive in his foreign policy. He did a number of things that can be seen as evidence that he was an imperialist.
Some of Napoleon III’s imperialism occurred relatively close to home. He worked to break free from the constraints that had been placed on France after its defeat in 1815 in the Napoleonic Wars. A good example of this is how he strengthened French influence in the Papal States, making sure that Italy could not annex them. Moving farther away, one of the most famous things that Louis Napoleon did was to try to set up something of a French imperial presence in Mexico. Mexico had been in the midst of internal conflict and Napoleon III (at the request of one faction in the conflict) placed a French-controlled emperor (Maximilian) on the throne in that country. Had this succeeded, it would have made Mexico, in effect, part of the French Empire. Still farther away, Louis Napoleon was aggressive in his policy towards Asia. He took control over some parts of Indochina during that time and pressed for more influence in China.
In all of these ways, and a few others, it is clear that Napoleon III was an imperialist.