Biden, who was frequently interrupted by chants of, “We love Joe,” painted a dark picture of the nation under Trump and recalled his decision to seek the presidency again in the aftermath of the 2017 clashes involving white supremacists in Charlottesville, Va. “Hate was on the march in America,” Biden said.

The president defended his economic record, saying he worked with Harris to rebuild the U.S. economy following the Covid-19 pandemic through a host of legislative measures, including a law to speed improvements to roads and bridges and cut the costs of prescription drugs. He said he and Harris had achieved an “extraordinary four years of progress.”

Biden said choosing Harris to be his running mate during the 2020 campaign was “the best decision I made in my whole career.” He described Harris as “tough,” “experienced,” and having “enormous integrity.”

Biden’s withdrawal from the top of the ticket in July meant the president spoke on the convention’s opening night—not in prime time on Thursday night—and he used the event to affirm Harris and make the case for her agenda.

The president was introduced by his daughter, Ashley Biden, who described his commitment to family, prompting Biden to dab his eyes with tissues as he took the stage. Earlier, first lady Jill Biden spoke of her husband’s strength and character and noted that she saw him “dig deep into his soul” to make the decision to step aside from the presidential race.

Biden’s announcement and swift endorsement of Harris put an abrupt end to weeks of party infighting over whether the 81-year-old leader should step aside following his disastrous debate performance against Trump. The party has rallied behind Harris and Biden received a warm outpouring from delegates—many of whom are relieved that the contest now looks far more competitive than it did before Biden made his decision.

During the night, Biden’s service was lauded by a number of speakers, including former Secretary of State Hillary Clinton , Rep. James Clyburn (D., S.C.), whose endorsement helped him seize control of the 2020 Democratic nomination, and Sen. Raphael Warnock (D., Ga.), whose words helped prompt expressions of gratitude from the hall.

Harris made a surprise appearance on stage Monday night to salute Biden. “Joe, thank you for your historic leadership, for your lifetime of service to our nation and for all you will continue to do. We are forever grateful to you.”

White House aides and longtime Biden advisers pushed back against any notion that the speech was a farewell, outlining plans for the president to support the Harris-Walz ticket in the weeks ahead.

“He still has a lot of ground in front of him and there will be an opportunity sometime later on to talk about his legacy,” said Ted Kaufman , a longtime Biden friend and adviser.

Monday night, however, served as a bit of a farewell for Biden, whose political career has spanned five decades. Biden attended his first Democratic National Convention in 1972 as he was seeking a long-shot bid to represent Delaware in the U.S. Senate. Family members and longtime friends from Delaware traveled to Chicago for the speech.

Inside the United Center, the arena will feature digital banners with Biden quotations, including, “History is in your hands. Let’s win this!” and “Spread the faith.”

Rep. Steven Horsford (D., Nev.), chair of the Congressional Black Caucus, said Biden’s appearance before the delegates would be one of “great respect, admiration and honoring 50 years of public service.”

“He came to the decision on his own and because of that, now you’re seeing a level of enthusiasm the likes of which I have not seen in any presidential campaign in my lifetime and I thought ’08 was it,” said Horsford, who offered a key endorsement of Biden before the Nevada caucus in 2020 and traveled recently with the president to Austin, Texas.

But outside of the president’s remarks, the convention will focus heavily on Harris’s background and a forward-looking message of where she would seek to take the nation. When former President Barack Obama addresses the convention on Tuesday night, Biden is scheduled to be in California for the remainder of the week taking a vacation. Harris’s running mate, Minnesota Gov. Tim Walz , will speak Wednesday night, followed by Harris on Thursday night.

Biden is the first sitting president not to seek re-election since President Lyndon Johnson in 1968. And his speech will offer parallels to addresses that Obama delivered in 2016 and former President Bill Clinton gave in 2000, both at the end of their second terms.

The White House has looked to move past any awkward appearances in the shift in programming, saying the president will reprise themes from his 2020 candidacy on restoring the soul of the nation and citing the need to preserve American democracy.

The president spent the weekend at Camp David, where he was joined by longtime adviser Mike Donilon , who has frequently collaborated with the president on the drafting of major speeches and returned from the campaign to help with the president’s final months in office.

Biden hasn’t deviated from the position that he could have beaten Trump. But he is more at peace with his decision to not seek re-election, his advisers have said, and has been encouraged by Harris’s momentum at the top of the Democratic ticket.

More than 200 people who worked for Biden either in the Senate or on one of his Delaware campaigns signed a letter thanking him for his service; the letter appeared in a full-page ad in Sunday’s edition of the News Journal, Biden’s hometown newspaper in Delaware. And for some longtime Biden allies, Monday’s convention speech—and his coming exit from the political stage—will be bittersweet.

Wrapping up his address, Biden pointed to his 50 years in public life. “America, I gave my best to you…I made a lot of mistakes in my career. But I gave my best to you.”

Write to Ken Thomas at ken.thomas@wsj.com