Weld was appointed a 2nd lieutenant in the 18th Massachusetts Regiment on January 27, 1862, at the age of 19. He was promoted to 1st lieutenant on November 1 of the same year and to captain on June 1, 1863.
On January 27, 1862, when he was 19 years old, Weld was commissioned as a 2nd lieutenant in the 18th Massachusetts Regiment. He was promoted to 1st lieutenant by November 1 of the same year and then became a captain on June 1, 1863.