My paternal great-uncle, Jack C. Sullivan, left for France aboard the Martha Washington from Hoboken, New Jersey. He held the rank of sergeant in the 4th Engineers, Co. E of the Fourth Division (Regular). Now I have a new question. When was he promoted? He had to have been promoted while still in the United States.
Unfortunately, his service record was part of those records destroyed in the 1973 fire at the National Personnel Records Center in St. Louis. When I requested his file, I was offered a copy of the final pay voucher for his unit. I learned he enlisted on 4 June 1917.
The 4th Engineers were formed at Camp Lewis in Washington. Jack sent home a letter to his sister reporting he had enlisted.
FamilySearch has a wonderful database, “United States, World War I, military muster rolls and rosters, 1916-1939.” They are not indexed but each regiment is listed in a pseudo-alphabetical order, starting with the 101st Ammunition Tr. 26 Div, 5th Company.
There are a total of 26 pages of lists of films. I needed to browse the pages to find the unit I was looking for.
Because Jack was in the 4th Engineers, I jumped to page 12 where I found several films with 4th listed. Here one film starts with the 4th Engineers Co. C and goes to the 5th Engineers Co. A. So, I know that Co. E. will be found within that digital film.
Moving through the pages, the officers are listed first, then the sergeants and corporals, and then the privates. Jack was found on image 451 as a private. I learned he joined the unit on 20 June 1917, along with the other men listed on the page. Here is the lower portion of the page, where he was listed at the bottom.
As I continue viewing each image of the muster rolls, I finally got to those from 31st of December 1917 to 28th of February 1918, where I found Jack has been promoted to Sergeant per Regimental Order No. 46 dated 21 February 1918.
These muster rolls continue until the units are back in the states. I found the last one Jack was listed in and it mentions the date he was mustered out.
The muster rolls can work in conjunction with morning reports, which are available at the National Personnel Records Center in St. Louis on microfilm. However, these muster rolls are available online at FamilySearch and can be viewed from our own computers.
 "U.S. Army Transport Service, Passenger List," database & images, Ancestry (http://www.ancestry.com), Martha Washington, Hoboken, NJ, 1918, Jack C Sullivan, Sgt, no. 568973.
 “The 1973 Fire, National Personnel Records Center,” National Archives (https://www.archives.gov/personnel-records-center/fire-1973).
 World War I Enlisted Men Final Military Pay Vouchers, 1917 - 1921, Multi-name final pay voucher, no. 202, 6 Aug 1919, Fort D.A. Russell, Wyoming, Capt P.O. Kowalski, quartermaster, p. 4, line 1, Jack C. Sullivan, Sergeant, 4th Engineers.
 "Anaconda Well Represented," The Anaconda Standard, 16 June 1917; online images, GenealogyBank (http://genealogybank.com : accessed 7 August 2013).
 “United States, World War I, military muster rolls and rosters, 1916-1939,” digital images, FamilySearch (https://www.familysearch.org/ark:/61903/3:1:3QHV-F3ZJ-29CX-F?i=443&cat=3378445), 4th Engineers, Co. E, 30 Apr 1917-30 Jun 1917, image 444, cover sheet; NARA RG 64, WWI Roster of enlisted men, 4th Eng Reg Co C – Co A 5th Engrs, reel 16.12, roll 1590.
 Ibid, image 451, privates, Jack C. Sullivan.
 Ibid, image 472, 31 Aug 1917 to 31 Oct 1917, Corporals, Jack C. Sullivan.
 Ibid, image 502, 31 Dec 1917 to 28 Feb 1918, Sergeants, Jack C. Sullivan.
 “Access to Morning Reports and Unit Rosters,” https://www.archives.gov/personnel-records-center/military-personnel/morning-reports-and-unit-rosters-access.