Yes, you can start a sentence with whereas.
Whereas, Arbor Day was first observed in Nebraska with the plantation
of more than a million trees, it is now observed throughout the nation
and the world
Both following clauses as above must be in same voice.
When I saw this sentence first time; I felt like it is a run on sentence, but whereas is a subordinating conjunction, which can be placed at the beginning and it still functions as a contrasting agent between following two independent clauses separated by commas.
While and whereas both can be used in this way. However, by contrast and on the other hand, must be used only to indicate contrast between two independent sentences separated by a full stop.
Few people believe that... . However, other argues that... .
Few people believe that... , whereas(while) other argues that...
refer the punctuation difference.
I have found this information through various online forums, and I am not a grammar guru.