William H. Seward, future Secretary of State, entered the first plea of insanity in the USA in 1847. His client was William Freeman, the murderer of John and Sarah van Nest, Sarah's mother and the couple's two year old son in 1847. Seward lost the case but on appeal won the right to fully argue the insanity defense in a new trial. Unfortunately, Freeman died in jail of tuberculosis before the case could be tried.
http://www.nycourts.gov/history/leg...story-new-york-legal-eras-people-freeman.html
Future Attorney General and then Secretary of War, Edwin Stanton made the first plea of "TEMPORARY insanity" in the case of Daniel Sickles' murder of Philip Barton Key in 1859. He succeeded in his argument.
http://www.historyandheadlines.com/first-successful-use-of-temporary-insanity-defense/
Edit - It is true that the original question did not include the word "temporary," nor did it say that the first insanity defense was successful. The official answer of William H. Seward is quite correct.
It is also true that we very recently had a question that dealt with Daniel Sickles' murder trial and Edwin Stanton's role in the temporary insanity defense that won acquittal for Sickles, which undoubtedly influenced a great many, if not all of our players to think that this question was about the same incident.
And I can tell you that it is true that when I enter "first plea of insanity as defense in USA" into my search engine, the very first source that comes up is the Wikipedia article on "insanity defense," and the search engine's brief summary includes the excerpt "The defense was first used by U. S. Congressman Daniel Sickles." By contrast, the nycourts.gov website cited as the source supporting the official correct answer is not among the first forty sources that my search engine came up with.
So, although we can all learn something from this question, I am going to accept Edwin Stanton as a correct answer.
I will not, however, accept Daniel Sickles as a correct answer. The question clearly asked for the name of a future member of Lincoln's Cabinet, and Sickles never served as such.
Hoosier