I explored on this topic a bit, It turns out that guitars were used in Rock 'n' Roll before mid 1950s, prior to them being used as lead instruments. The earliest I could find was late 1920s from this article, which says,
"Crazy About My Baby" by Blind Roosevelt Graves and brother Uaroy, recorded in 1929, was a rhythmic country blues with small group accompaniment. Researcher Gayle Dean Wardlow has stated that this "could be considered the first rock 'n' roll recording". The brothers also recorded rhythmic gospel music. The Graves brothers, with an additional piano player, were later recorded as the Mississippi Jook Band, whose 1936 recordings including "Skippy Whippy", "Barbecue Bust" and "Hittin' The Bottle Stomp" were highly rhythmic instrumental recordings which, according to writer Robert Palmer, "..featured fully formed rock and roll guitar riffs and a stomping rock and roll beat".
If Guitars were used in Rock n Roll before 1950s?
Since when guitars were used in Rock 'n' Roll music?
Apparently from the beginning of Rock 'n' Roll itself. However, right now, I cant cite a specific song where guitar was used or an exact year when it was used first.