Lest anyone think that Mikulski's tenure on Capitol Hill has been only about milestones, it has also been one of historic legislative accomplishments.
She fought hard for passage of the Lilly Ledbetter Fair Pay Act, a bill to guarantee equal payment for women, and railed against President George W. Bush in April 2008 when he threatened to veto the measure.
"But now the president has issued a veto threat. He said this bill is going to 'impede justice.' That is baloney," she said. "This bill doesn't impede justice, it restores justice. It reinstates a fair rule for both workers and employers. He said it is going to mess up the process. This bill does not slow down the process. It gives people a way of getting into the process.
"Now it is time to fight for Lilly Ledbetter and the 150 million women in her position," she argued.
And Mikulski championed Rosa's Law, a citizen advocacy measure that banned the use of the term "mentally retarded."
"On behalf of all the children of the United States of America who are labeled, stigmatized and bear a burden the rest of their lives because of the language we use in the law books, my law simply today changes the phrase 'mentally retarded' to 'intellectually disabled' and we do it in health, education and labor policy without in any way negatively impinging on either the educational or other benefits that these children are entitled to," she said in a floor speech in the fall of 2009.
Becoming the longest-serving female senator won't be the last of Mikulski's firsts, either. Come March 17, 2012, she will become the longest-serving woman in the history of Congress.