Merwin was born on 6 October 1874 in Evanston, Illinois to Ella B. and Orlando H. Merwin. His father was the postmaster of Evanston. In 1901, Merwin married Edna Earl Fleshiem.
Archibald Alexander (April 17, 1772 – October 22, 1851) was an American Presbyterian theologian and professor at the Princeton Theological Seminary. He served for 9 years as the President of Hampden–Sydney College in Virginia and for 27 years as Princeton Theological Seminary's first principal from 1812 to 1840.