It doesn't come before mountains usually, but it does come become before mountain ranges:
Mount Everest is in the Himalayas.
(There are a few exceptions- European mountains that have 'the' in their original language can take the article, like 'the Matterhorn'.)
Do you mean just geographical terms and place names or in a wider sense? Because the rules for the wider sense are very complex and a word can take an article or not according to the meaning.