Alex Padilla has been named to replace Vice-President-elect Kamala Harris in the US Senate by California Governor Gavin Newsom, filling the remainder of her term in the upper house.
He is the first Latino to represent California in the house and a close ally of Mr Newsom.
The son of Mexican immigrants, he became California's first Latino secretary of state in 2015.
He is due to hold the Senate seat until 2022 when Ms Harris's term ends.
In a statement, Mr Padilla pledged to "work each and every day to honour that trust and deliver for all Californians".
"From those struggling to make ends meet to the small businesses fighting to keep their doors open to the health care workers looking for relief, please know that I am going to the Senate to fight for you. We will get through this pandemic together and rebuild our economy in a way that doesn't leave working families behind," he added.
I am humbled and honored to serve as California’s next United States Senator. Thank you, @GavinNewsom, for entrusting me with the role of defending the dream for the people of California. https://t.co/bCwf05C3hm pic.twitter.com/Fn5iNl6ldS— Alex Padilla (@AlexPadilla4CA) December 22, 2020
The son of a cook and a domestic cleaner who grew up in a Los Angeles neighbourhood rife with gangs and drugs, Mr Padilla, 47, was an engineer - having graduated from the prestigious Massachusetts Institute of Technology - before entering politics when anti-immigrant rhetoric was high in California.
"Alex Padilla worked his way from humble beginnings," Mr Newsom said in a statement, in which he further described Mr Padilla as "a warrior for voting rights and the American Dream".
"Through his tenacity, integrity, smarts and grit, California is gaining a tested fighter in their corner who will be a fierce ally in [Washington] DC, lifting up our state's values and making sure we secure the critical resources to emerge stronger from this pandemic," he added.
His appointment will make history. But the @AlexPadilla4CA I know is far more interested in changing history -- especially for the working men and women of our state and country.— Gavin Newsom (@GavinNewsom) December 22, 2020
I can think of no one better to represent the state of California as our next United States Senator. pic.twitter.com/xiAzpTS42Y
Vice-president-elect Harris also welcomed the appointment.
However, some California Republicans were less impressed with the appointment, including Harmeet Dhillon, a conservative civil rights lawyer and member of the Republican National Committee, who tweeted:
One of the worst election officials in the nation will go to DC, unelected!!! A fitting exit to his failed tenure. Remember the time he said door to door ballot harvesting by strangers was cool during a pandemic shutdown when businesses and churches were shuttered? Good times. https://t.co/g6pLSnwfFT— Harmeet K. Dhillon (@pnjaban) December 22, 2020