Frederick Douglass makes his purpose in publishing the open letter to Thomas Auld clear in the final paragraph. Auld was his former slave master, from whom he had run away exactly ten years before he wrote the letter on 3 September 1848. Having written nine long paragraphs of narrative and exposition, Douglass concludes by writing:

I intend to make use of you as a weapon with which to assail the system of slavery—as a means of concentrating public attention on the system, and deepening their horror of trafficking in the souls and bodies of men. I shall make use of you as a means of exposing the character of the American church and clergy—and as a means of bringing this guilty nation with yourself to repentance.

Douglass says that he entertains no malice towards Auld personally ("There is no roof under which you would be more safe than mine") and that he would regard it as a privilege to show Auld how men ought to treat each other. This was generous of him, since Auld was a cruel master. In fact, the two did meet almost thirty years later in 1877, after the abolition of slavery, when Auld was on his deathbed and told Douglass that in his place, he would also have run away.

Besides his public purpose, Douglass had a further personal reason for writing the letter, which he discloses in paragraph eight:

At this moment, you are probably the guilty holder of at least three of my own dear sisters, and my only brother in bondage. These you regard as your property. They are recorded on your ledger, or perhaps have been sold to human flesh mongers, with a view to filling your own ever-hungry purse. Sir, I desire to know how and where these dear sisters are. Have you sold them? or are they still in your possession? What has become of them? are they living or dead? And my dear old grandmother, whom you turned out like an old horse, to die in the woods—is she still alive? Write and let me know all about them.

He points out that if his grandmother is still alive, she can be no possible use to anyone as a slave since she must be almost eighty and asks Auld to send her to him, so that he can care for her in her old age.