It’s almost Pavlovian: American cider writers are incapable of publishing a cider history or how-to without stating that John Adams, Doer and Patriot, drank cider for breakfast. They might even include how much he drank (a tankard full being the most popular) or why he did so (everything from cider being a cure-all to it being the key to long life). If you search the phrase “John Adams drank cider every morning” (there we did it for you) you’ll get over 73,ooo hits. You’ll find Adams’ cider habit in everything from the Woodchuck Hard Cider Company’s Five Tidbits of Presidential Hard Cider History post to the traveling exhibition American Spirits: The Rise and Fall of Prohibition, who included it on this text panel:
Adams’ breakfast cider is mentioned so often and in so many variations that it made me wonder if there was more to the story.
As it turns out, there’s less to the story. At least according to Adams’ own diary. He mentions his morning cider on only two occasions:July 26, 1796In conformity to the fashion I drank this Morning and Yesterday Morning, about a Jill of Cyder. It seems to do me good, by diluting and dissolving the Phlegm or the Bile in the Stomach. July 28, 1796I continue my practice of drinking a Jill of Cyder in the Morning and find no ill but some good Effect. 
Besides indicating this was something new for him (as of 1796), he also notes that these “medicinal” drams weren’t all that large. Adams clearly states he has a jill (or gill) of cider. According to the 1799 edition of Johnson’s Dictionary (see quartern), a gill equals one quarter of a pint.
So yes, for what seems to be a short time he drank a small amount of cider each morning for medicinal purposes. By his own account it certainly wasn’t a tankard and he didn’t think it would cure all his ails or help him live longer, though drinking a small amount of cider on a nearly-empty stomach every morning would certainly have a “good Effect.”
1. Both quotations are from the John Adams diary 46, 6 August 1787 – 10 September 1796, 2 July – 21 August 1804 [electronic edition]. Adams Family Papers: An Electronic Archive. Massachusetts Historical Society. http://www.masshist.org/digitaladams/. There are no other mentions of his morning cider habits before or after these two.