The page http://www.hymnary.org/text/o_savior_let_thy_kingdom_come seems to be in error. It states the title of the hymn is "Let Me Not Forget." However, that is the title of Pilson's other hymn, "Let Me Nor Forget," which appears in in Alexander's Gospel Songs & Solos, 1917 (no page numbers in printed version, but around page 158 in PDF file).
We cannot find "O Savior, let thy kingdom come," in the Republican Hymn Book of 1845, which itself states that Thomas Herbert is the author of all its content. We did not find any other author names mentioned there.
O Savior, let thy kingdom come
I fixed this. "O Savior, let thy kingdom come" is in "The Republican Hymn Book" but it is clearly written by Thomas Herbert.