On August 8, 1911, U.S. Patent No. 1,000,000 issued to F. H. Holton on a Vehicle Tire:

While this patent is numbered 1,000,000 is not really the one millionth U.S. Patent. The Patent Office didn’t start numbering patents until July 13, 1836, but the U.S. Patent Office starting issuing patents on July 31, 1790. It is estimated that 9,957 patents issued between July 31, 1790, and July 13, 1836, and these patents are known as the X patents. and are numbered accordingly. No one knows the exact number because the records of the X patents were destroyed in the Patent Office fire of December 15, 1836, ironically while in storage while a more fireproof storage could be constructed. Only about 2800 of the 9,957 X patents could be reconstructed, although the effort continues, the most recent addition occurring in 2012, when patent X7997 on “Rope Making, Called an Iron Tail”, was found in the inventor’s (Daniel Treadwell) at Harvard University.

So, the millionth U.S. patent may be U.S. Patent No. 990043, issued April 11, 1911, to C.T. Hanson on a Machine for Making Tubes from Fibrous Materials: