Bipolar disorder is not so much discovered as it did exist throughout time. Dated back to Ancient Greece where Aretaeus of Cappadocia, who was a physician and philosopher in the time of Nero or Vespasian (first century AD) had observed patients that would dance and play all night for days even. Then without notice change to dull and somber and weeping.

It has always been there, just not yet labeled as such. 
In the early nineteen hundreds the term was called “Manic Depressive” in order to give a term to the manic side of mania and its counterpart depression

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 In 1854, Jules Baillarger (1809–1890) and Jean-Pierre Falret (1794–1870) both presented psychiatric terms and writing about the illness called ~dual insanity ~ 

It was In the early 1900s the eminent German psychiatrist Emil Kraepelin (1856–1926) coined the term that would be used to describe what we know as bipolar today…