The ceremony took place on the steps of the Virginia Capitol.
Northam, who was previously Virginia’s lieutenant governor, last November defeated former Republican National Committee Chair Ed Gillespie. Northam succeeds former Gov. Terry McAuliffe.
Democrat Justin Fairfax, who is the second African American elected to statewide office in Virginia, on Saturday was sworn in as lieutenant governor. Democrat state Attorney General Mark Herring was also sworn in for a second term.
Northam, Fairfax and Herring support LGBT rights.
Northam on Saturday signed an executive order that bans discrimination against state employees based on their sexual orientation, gender identity and other factors.
McAuliffe in 2014 signed an identical executive order on his first day in office.
Northam in his inaugural speech referred to efforts to combat anti-gay discrimination in Virginia. Equality Virginia is among the organizations that were part of the Northam Inaugural Committee.
“Virginians didn’t send us here to be Democrats or Republicans,” said Northam. “They sent us here to solve problems.”
Human Rights Campaign Senior Vice President for Policy and Political Affairs JoDee Winterhof in a statement said Northam “consistently campaigned on a platform of equality for all Virginians.” She added Virginians on Election Day “resoundingly rejected the Trump-Pence politics of hate, fear and discrimination.”
“There is so much work to do in Virginia, and we look forward to working closely with Governor Northam to advance LGBTQ equality across the commonwealth,” said Winterhof.
The inauguration took place three days after the 2018 General Assembly began.
State Del. Danica Roem (D-Manassas) is the first openly transgender person ever seated in a state legislature in the country. State Del. Dawn Adams (D-Richmond) is the first out lesbian ever elected to the Virginia General Assembly.
State Sen. Adam Ebbin (D-Alexandria) and state Dels. Mark Levine (D-Alexandria) and Mark Sickles (D-Fairfax County) are openly gay.
The Republicans have slim majorities in both houses of the General Assembly.