Free franked envelope sent by Lincoln's private secretary, John Nicolay, to the Judge Advocate General.
Lincoln won the presidency in November 1860, and was inaugurated on March 4, 1861. His first act as commander and chief was the appointment of a personal secretary, John Nicolay. This envelope (at left) sent "from the President of the United States" was signed by Nicolay as Lincoln's private secretary. Prior to his work at the White House, Nicolay worked as a journalist and clerk for the Illinois Secretary of State. Throughout the majority of Lincoln's presidency, he aided the president with various diplomatic affairs and organized many social events. Nicolay's service ended at the beginning of Lincoln's second term, just a few weeks before his assassination at Ford's Theatre.
His first act as commander and chief was the appointment of a personal secretary, John Nicolay.
source-https://arago.si.edu/exhibit_246_10.html
source- wiki via John George Nicolay.
Sign John G. Nicolay's appointment as private secretary. (Nicolay to Bates, 5 March 1861, John G. Nicolay Papers, Library of Congress, Washington, DC.)
Very interesting question. I find his first official anything was to make his nominations for his cabinet picks on March 5, 1861. Although I can not find the exact date, Lincoln's appointment of Johann (John) George Nicolay as his private secretary, was the first official act of his new administration.
In 1861, Lincoln appointed Nicolay as his private secretary, which was the first official act of his new administration John George Nicolay (February 26, 1832 – September 26, 1901)
Source:
The answer was amazingly hard to find--I started with Godwin's Team of Rivals, which I just finished--no luck there on his first official act. Several sources said his first official act was to announce his cabinet. I did find two sources for the answer I've given.
Confirmed by wikipedia article, "John George Nicolay":
"In 1861, Lincoln appointed Nicolay as his private secretary, which was the first official act of his new administration."
Per Goodwin, op. cit., p. 264, Lincoln had hired Nicolay as his assistant right after the Republican nominating convention. However, that obviously was not an official presidential act.