It is generally believed that Loraine Boettner was the first to coin the TULIP acrostic as a mnemonic device for easier memorization in his 1932 book, The Reformed Doctrine of Predestination.
Recently, however, Wayne Sparkman, Director of the PCA Historical Center, has uncovered documentation which shows that the mnemonic device was used before Boettner, although in a slightly different form.
William H. Vail, "The Five Points of Calvinism Historically Considered," The New Outlook, Vol. 104 (May-August 1913), p. 394:
Some eight years ago [ie., 1905] I had the privilege of hearing a popular lecture, by Dr. [Cleland Boyd] McAfee (1866-1944), of Brooklyn, upon the Five Points of Calvinism, given before the Presbyterian Union of Newark, New Jersey, which was most interesting as well as instructive. To aid the mind in remembering the Five Points, Dr. McAfee made use of the word Tulip, which, possessing five letters, lends itself nicely to the subject in hand, especially as it ends with the letter P, as will be seen later.
Taking the five letters, Dr. McAfee used them as follows:
1st, T stands for Total Depravity.
2d, U " " Universal Sovereignty.
3d, L " " Limited Atonement.
4th, I " " Irresistable Grace.
5th, P " " Perseverance of the Saints.
It will be noticed that McAfee's acrostic employs "U" to represent "Universal Sovereignty," while Loraine Boetter's "U" represents "Unconditional Election." Boetter also refers to "Total Inability" rather than "Total Depravity." Nevertheless, it appears, based on this research, that credit should go to Dr. McAfee for the earliest usage of the TULIP acrostic to convey the Five Points of Calvinism, and to Mr. Sparkman for bringing this fascinating bit of history to our attention.