In his recent article, Time to Kill: Euthanizing the Requirement for Presidential Approval of Military Death Sentences to Restore Finality of Legal Review, MAJ Toman writes:
Since the approval of the UCMJ in 1950, ten service members have been tried and executed. The last Soldier was executed in 1961. See Captain Cody Weston, United States v. Loving: The Resurrection of Military Capital Punishment, 77 OR. L. REV. 365, 369–70 (1998) (citing Cynthia Swarthout Conners, The Death Penalty in Military Courts: Constitutionally Imposed?, 30 UCLA L. REV. 366, 369 n.18 (1982)). But see DWIGHT H. SULLIVAN, EXECUTIVE BRANCH CONSIDERATIONS OF MILITARY DEATH SENTENCES 137 (2002) (stating the number as twelve executions and fourteen commutations based on a memorandum from Attorney General Robert F. Kennedy in 1961); EUGENE FIDELL, EVOLVING MILITARY JUSTICE (2002).195 Mil. L. Rev. 1, 7 n.14 (2008).
The military has actually executed 12 servicemembers since President Truman signed the UCMJ into law on 5 May 1950, though what MAJ Toman wrote is literally true because 2 of the 12 were tried before 5 May 1950. In May 1949, three Air Force general courts-martial convicted Private Herman Dennis, Staff Sergeant Robert Burns, and Private Calvin Dennis of rape and premeditated murder and sentenced each to death. An Air Force Board of Review affirmed all three death sentences. United States v. Dennis, 4 C.M.R.(A.F.) 872 (1949); United States v. Burns, 4 C.M.R.(A.F.) 907 (1950); United States v. Dennis, 4 C.M.R.(A.F.) 930 (1949). The Judicial Council affirmed the three death sentences as well. Dennis, 4 C.M.R.(A.F.) 888-904 (1950); Burns, 4 C.M.R.(A.F.) 923-27 (1950); Dennis, 4 C.M.R.(A.F.) 943-53 (1950). In 1951, President Truman approved two of the death sentences and commuted the third. Dennis, 4 C.M.R.(A.F.) at 907 (1951) (approving death sentence); Burns, 4 C.M.R.(A.F.) at 930 (1951) (approving death sentence); Dennis, 4 C.M.R.(A.F.) 956 (1951) (commuting death sentence). Habeas litigation in the two approved cases ultimately went to the Supreme Court, which denied relief. Burns v. Wilson, 346 U.S. 137 (1953). The two Airmen were hanged at Northwest Military Air Field, Guam, on 28 January 1954. See Airmen Hanged in Guam, N.Y. Times, Jan. 28, 1954, at 7.
Those two executions were in addition to the 10 executions of Soldiers convicted under the UCMJ. (O'Brien - 1954; Beverly - 1955; Suttles - 1955; Riggins - 1955; Edwards - 1957; Moore - 1957; Ranson - 1957; Thomas - 1958; Day - 1959; Bennett - 1961).
So there have been 12 executions of servicemembers in the UCMJ era, though only 10 of those 12 were tried under the UCMJ, the other two having been tried under the Elston Act.