There are two well-known meanings of "whereas", which are roughly "given that" (legal) and "in contrast to" (common). The Douglas Harper, Online Etymology Dictionary only addresses the legal usage, which appears to be old. If that is the original usage, where did the now common usage come from? (Perhaps as a replacement of whyle as?)
The OED has a citation for the (legalese) meaning of 'whereas', in the sense of 'in view of':
(1426-27) W. Paston in Paston Lett. & Papers (2004) I. 10 Where as þe seyd William Paston, by assignement and commaundement of þe seyd Duk of Norffolk..was þe styward of þe seyd Duc of Norffolk.
An early citation for the usage 'in opposition to the principal clause in the sentence' is:
(1535) Bible (Coverdale) 2 Esdras vii. 5 There are layed vp for vs dwellynges of health & fredome, where as we haue lyued euell.
So it appears that both meanings obtained from fairly early on, and the latter use is not a modern development.