Mathematics is, as Jim Reynolds said, singular despite being plural in form. There are various other words like it, including many other words ending in -ics such as physics,dynamics, ethics, linguistics, metaphysics, optics, economics. The Oxford English dictionary summarizes the situation for words with this suffix as follows:
The names of sciences, even though they have the form in -ics, are now
construed as singular, as in ‘mathematics is the science of quantity;
its students are mathematicians’; in recent times some writers,
following German or French usage, have preferred to use a form in -ic,
as in dialectic, dogmatic, ethic, metaphysic, static, etc. Names of
practical matters as gymnastics, politics, tactics, usually remain
plural, in construction as well as in form.
There may be some variation for some of these words. Personally, I agree with the OED about politics are and tactics are, but I think I would be inclined to say gymnastics is (when referring to the discipline; I would use the plural in a phrase like "her verbal gymnastics are not appreciated"). Gymnastics are and gymnastics is seem to be fairly close in frequency on the Google Ngram Viewer.