Gainey was sworn in by Allegheny County Common Pleas President Judge Kim Berkeley-Clark.
During his inaugural address, Gainey thanked his family for their unwavering support and all the voters that elected him into office.
“Without this city, I would not be who I am today, and I want you to know that I will never take your support or vote for granted,” Gainey said.
“I understand that your trust and your confidence is not permanent but something that must be earned and protected every single day,” he added.
Gainey pledged that his administration will be "progressive, principled and always on the side of the people."
"My promise to you is that we will work to make Pittsburgh the Pittsburgh you voted for—a city where economic opportunity is abundant for everybody, a city where affordability isn't a luxury, a city that is prepared to lead into the future," he said.
Pennsylvania Gov. Tom Wolf (D) called Gainey a "strong leader" who has the "determination to lift up people in his community.”
"Mayor Gainey’s focus on improving opportunity for everybody is going to put Pittsburgh and the people who call it home on a path to even greater things," Wolf said.
A graduate of Morgan State University, Gainey worked six years as a legislative aide to Pennsylvania State Representative Joseph Preston Jr., and as a special projects manager under Pittsburgh Mayor Tom Murphy. He gained a reputation for championing economic development in the East Liberty section of Pittsburgh where he was born.
In 2010, he became the first African-American to serve as chairman of the Pittsburgh City Democratic Committee.
After challenging Preston on two separate occasions, in 2012, Gainey defeated his former boss, in a Democratic primary. Since 2013, Gainey has represented the 24th district.
To become the city’s first mayor, Gainey defeated incumbent Bill Peduto in the primary and went on to win the general election against Republican candidate Tony Moreno.
Gainey paid tribute to all those who came before him to make his victory possible.
"Thank you for your trust in me. I promise I will make you proud," Gainey said. "I am here today as a physical manifestation of a labor of love. I stand here because of names of people I don't even know who struggled for me, who fought for me."