Newsom, 43, a Democrat, served seven years as mayor of San Francisco. He is known for implementing the Care Not Cash program on homelessness and allowing same-sex marriages in the city during a brief period in 2004.
A political moderate, Newsom was sometimes criticized by progressives for his ties to the city's business community and developers.
Newsom delayed his swearing-in as lieutenant governor by one week in order to ensure that the prior, progressive-dominated Board of Supervisors did not appoint an interim mayor with a far-left political agenda.
After much political jockeying, City Administrator Ed Lee, whom Newsom approved of, was selected by the board on Friday. Four new board members were sworn in Saturday, and the board is expected to approve Lee's yearlong interim term on Tuesday.
In a surprise move on Sunday, Newsom appointed police Chief George Gascon to be the city's new district attorney to replace Kamala Harris, who is now attorney general of California.
Newsom is scheduled to be sworn in as lieutenant governor at 1 p.m. in the Senate chamber at the state Capitol. He will now be second-in-command to Democratic Gov. Jerry Brown.