John George Nicolay (February 26, 1832 – September 26, 1901) was a German-born American author and diplomat who served as private secretary to U.S. President Abraham Lincoln and later, with John Hay, co-authored Abraham Lincoln: A History, a ten-volume biography of the 16th president.

It appeared in The Century Magazine serially from 1886 to 1890 and was issued (1890–94) in book form as 10 volumes, together with the two-volume Complete Works of Abraham Lincoln.

The book was based on envelopes of material that Nicolay had collected but been unable to use in the biography of Lincoln that he wrote with Hay.

Helen Nicolay wrote in the preface to the book that the envelopes contained "miscellaneous notes, personal jottings, private letters, and newspaper clippings.

[9] Historian Joshua M. Zeitz writes, "Above all, Nicolay and Hay created a master narrative whose influence would ebb and flow over the years but that continues to command serious scrutiny and engagement....

[11] Poor health had forced Nicolay to resign as Marshal of the Supreme Court, and he suffered from a wide range of ailments in his final years.

In the 1988 NBC mini-series Lincoln, based on Gore Vidal's book, Nicolay is portrayed by actor Richard Travis.