According to the Metropolitan Museum of Art, the Mask of Agamemnon was discovered in 1876 by Heinrich Schliemann at Mycenae, and was actually "one of several gold funeral masks found laid over the faces of the dead buried in the shaft graves of a royal cemetery" (see reference link). As this article goes on to state, the mask would have, in all probability, been originally "raised from a single sheet of gold, just thick enough to hold its form without any waste of the precious metal."
Note, however, that much of what might be claimed or stated about this mask and its implications should be held, at least to some degree, in a tentative fashion. Indeed, we can't even truly say whether the mask was actually associated with Agamemnon himself (which should not be surprising, given that we cannot truly know whether the legendary Mycenaean king even existed at all). History as we now understand it tends to rely on the presence of a large-scale written record, and where such a record is lacking, there will always be an increased degree of historical uncertainty which must be accounted for. Of course, this should not be used to dismiss the value of archaeological discoveries such as this one and the insights they provide (nor should we dismiss the written record that the Mycenaeans did leave behind). At the same time, however, we should be aware of the limitations that remain.
That being said, I think several conclusions can be safely be presented, based primarily on the discovery of these masks. First, these artifacts lend further evidence to validate the claim that Mycenae's political culture was of an aristocratic nature (with these masks likely belonging to the privileged, ruling class). Likewise, based on the evidence of such artifacts alone, it should be safe to expect the Mycenaeans to have possessed complicated funerary rites and rituals. Even so, much of the specific detail would remain elusive, and more evidence is required to make more substantial claims and interpretations.