Allen served in the Illinois 12th Cavalry during the Civil War, and at one time served under General George Custer. Most of his service was near the Gulf in New Orleans, Mobile, Baton Rouge, etc.167

1870 Census, Ulysses, Butler Co., NE
48/48 (2 homes from brother James, 4 from brother Sumner)
Darnell, Allen, 26, farmer, real estate value $2000, personal estate value $1000, b. IL
Martha, 17, b. OH

Is this coincidence, or was he also known as Adam?
1880 Census, Kearney Twp., Clay Co., MO
Darnell, Adam, 36, farmer, b. IL, father b. KY, mother b. KY
Martha, 37, wife, keeping house, b. OH, parents b. OH
Ella, 9, dau., b. NE
George, 7, son, b. NE
Mary, 4, dau., b. IA
John, 1/12, May, son, b. MO

Death Certificate gives parents as Wm. Darnell, b. North Carolina, and Pricilla Thurman, b. VA or N. Carolina

Obituary, Liberty (MO) Tribune:
(from the Liberty Tribune)
“Allen Darnell died at the home of his daughter, Mrs. John T. Liggett, northwest of Liberty, at 8:00pm, Thursday, February 9, 1922. Mr. Allen Darnell fell asleep at the age of 78, having been born October 8, 1843.
Before he had reached the age of 19 he enlisted in Company E of the 12th Illinois Cavalry, and served until the end of the war.
He was married to Miss Mick, and to this union were born 3 sons and 3 daughters, all of whom were present at his burial except William who was sick at his home near Randolph.
His companion passed away April 19, 1903, and during these 19 years he patiently waited in loneliness of heart the meeting again. Often he has told the writer, who has been his friend during 43 years, that he could never lose the sadness and longing of his heart until he once more should find the bride of his young manhood in the better land.
He was a good father to his children, and they loved him with strong and tender devotion. We laid him to rest beside his wife in Providence Cemetery after services by the writer at the grave.
May the brave old soldier rest in peace.”