She hopes to win Senator Barbara Boxer's seat after Boxer retires in 2016.
In 1997, Harris worked as a deputy district attorney in Alameda.
In 2007, she was re-elected district attorney of San Francisco.
In 2017, she hopes to become the first African American senator from California.
It's a rapid rise that doesn't surprise her friends.
"She tends to concentrate on the things that have real importance and not get distracted," said Wilkes Bashford, a friend of Harris.
In Harris' Facebook announcement Tuesday morning, she says: "I want to be a fighter for families feeling the pinch of stagnant wages and diminishing opportunity, for students burdened by predatory lenders and skyrocketing tuition, for our immigrant communities and for our seniors."
Former San Francisco Supervisor Matt Gonzalez considers Harris the frontrunner.
"She campaigns hard, people who meet her like her and she has ideas," said Gonzalez.
Over the weekend, former Los Angeles Mayor Antonio Villaraigosa said he may run.
Representative Loretta Sanchez says she's "seriously considering" a senate bid.
A spokesperson for Representative Eric Swalwell says he'll make a decision by the end of this month.
Notably absent from the list is Lieutenant Governor Gavin Newsom.
"I think if Newsom wanted to run for this, it would have been very difficult to defeat him," said John Burton, the California Democratic Party Chairman.
Another wildcard is San Francisco billionaire environmentalist Tom Steyer. He posted an article in the Huffington Post that reads in part: "Washington needs to be shaken up. We need climate champions who will fight for the next generation. I will decide soon."
California AG @KamalaHarris announces campaign for US Senate, requests donations. A SF billionaire could be her biggest challenger.— Tiffany Wilson (@TWilsonTV) January 13, 2015
Forbes estimates Steyer's net worth at $1.6 billion.
"It would spice the mix up considerably, he's very well thought of, he can self-finance," explained Burton.
Perhaps that's one reason Harris is already fundraising.
Harris, 50, the daughter of an Indian mother and black father, was elected California attorney general in 2010. She was the first woman and the first minority to serve as California's top prosecutor. Harris is also a friend of President Barack Obama's and attracted national attention when she helped negotiate a settlement with major mortgage lenders and secured extra funding for California.
She has been widely viewed as an eventual candidate for governor or U.S. senator.
The Associated Press contributed to this story.