Johnson said in a statement that the decision to resign was "entirely personal and the result of recently discovered health issues."
"I believe that sunshine is the best disinfectant in the political process, and I am confident that the good work of the commission will continue," he said in a statement. "My fellow commissioners and I have worked together in a non-partisan way to interpret and enforce the Political Reform Act and I am proud of what we have accomplished over the past three years."
Johnson, a Republican who served in the state Legislature for more than 25 years, was appointed to the commission in February 2007.
Gov. Arnold Schwarzenegger may name a replacement to fill out the remainder of Johnson's term, which ends Jan. 31, 2011. The appointee cannot have the same political party registration as Schwarzenegger's other pick for the commission, Democrat Timothy A. Hodson.
Schwarzenegger issued a statement thanking Johnson for working "tirelessly to promote and protect the integrity of representative government in California."
"He has demonstrated true talent and commitment to the people of California during his long and distinguished public service career and I wish him the absolute best for the future. His leadership will surely be missed," he said.
Johnson's resignation is effective April 30. Read the full letter of resignation here.
PHOTO CREDIT: Fair Political Practices Commission Chairman Ross Johnson talks to The Bee's Capitol Bureau on April 17, 2007. Sacramento Bee file photo/ Brian Baer