Johnson authored a bill this year to legalize marijuana. It never made it out of committee.
Her bid for U.S. Senate means she is giving up a chance to run for re-election to her present office.
Coburn announced in January he would retire with two years left on his term. While Johnson is the first major Democratic candidate to declare for his post, the field on the Republican side includes U.S. Rep. James Lankford, state Rep. T.W. Shannon and former state Sen. Randy Brogdon.
In 1990, David Boren became the last Democrat to win a U.S. Senate seat in Oklahoma.