The Mercury selection process started on February 2, 1959 with the first meeting at the Pentagon, so I will keep 1959 as the rank evaluation year, which results in the following ranks. (*) denotes some uncertainty in the rank (see remarks below the table).
||Rank in 1959
||Date of birth
||Malcolm Scott Carpenter
||May 1, 1925
||Leroy Gordon Cooper, Jr.
||March 6, 1927
||John Herschel Glenn, Jr.
||July 18, 1921
||Virgil Ivan Grissom
||April 3, 1926
||Walter Marty Schirra, Jr.
||March 12, 1923
||Alan Bartlett Shepard, Jr.
||November 18, 1923
||Donald Kent Slayton
||March 1, 1924
- Carpenter was a lieutenant (JG) in 1951. I did not find a source for his promotion to Lieutenant, but typically promotion to LT is after 2 years, so it seems likely het got promoted, given that he got recommended by his superior to attend the USN Test Pilot School in 1954.
- Cooper was a flight commander in 1954, which was typically a role filled by captain.
- Glenn got promoted to Lieutenant Colonel in 1959.
- Grissom went to Edwards AFB as a Captain, just before being selected for the Mercury program.
- Schirra was promoted from Commander to Captain after his Mercury flight in 1962.
- Shepard was promoted from Commander to Captain after his Mercury flight in 1961.
- Slayton resigned from the USAF as Major in 1963, when he was grounded due to his heart condition (discovered in 1959, before he got to fly on Mercury).
There is some uncertainty here: Carpenter may have been promoted more than once between 1951 and 1959. Also Cooper may have been promoted between 1954 and 1959.
- Glenn, Schirra and Shepard were in pay grade O-5 and thus had highest seniority.
- Slayton was in pay grade O-4.
- Carpenter, Cooper and Grissom were in pay grade O-3 and had lowest seniority.
Glenn joined the Navy in 1942, well before Schirra and Grissom, so it is likely he reached O-5 the first, making him the most senior.
Carpenter and Cooper joined their respective branches in 1949, while Grissom joined the Air Force only in 1950. In addition, Grissom reached the rank of Captain just before the start of the Mercury program, while Cooper and Carpenter may have been promoted beyond the last rank I was able to verify. This makes it likely that Grissom reached O-3 the latest and thus him the least senior.
However, as I was unable to find exact dates of the men's promotions, this is speculation.