On the first full day of his US visit, Pope Francis also told his bishops to work to ensure the sex abuse scandal that has rocked the Catholic Church never happens again.
The White House receives many high-profile guests, but they don't usually arrive in a Fiat 500.
The Pope did - in a gesture of simplicity.
He was greeted by US President Barack Obama and his wife, Michelle.
"Holy Father, on behalf of Michelle and myself, welcome to the White House."
About 15,000 people gathered in bright sunshine on the White House lawn to welcome the 78 year-old, who's on his first visit to the world's richest nation.
Amid the fierce debate in the US about illegal immigration, the Pope described himself as the child of an Italian family who migrated to Argentina.
"As the son of an immigrant family, I am happy to be a guest in this country, which was largely built by such families."
The Pope and President are at odds over abortion and gay marriage, but agree on some issues, such as climate change.
The Pope has praised the President's efforts to cut carbon emissions.
"Mr President, I find it encouraging that you are proposing an initiative for reducing air pollution, accepting⦠accepting the urgency."
The President said the Pope has reminded people they have a "sacred obligation" to protect the planet.
"We support your call to all world leaders to support the communities most vulnerable to a changing climate and to come together to preserve our precious world for future generations."
The pair held about 40 minutes of private talks.
Asked whether the Pope's comments on climate change would influence debate over the issue in the US, White House spokesman Josh Earnest said it's too soon to know.
"Listening to the pope's comments I think it was clear that he was speaking with a passion and a conviction about the need to act on climate issues that was deeply rooted in his faith. So he wasn't seeking to deliver a political message, he was speaking from the heart."
The Pope disappointed some of America's 70 million Catholics by deciding not to meet any victims of sex abuse by priests.
But he didn't avoid the issue altogether.
At the Cathedral of Saint Matthew, the pontiff told US bishops the scandal that has rocked the Catholic church should never be repeated.
"I realize how much the pain of recent years has weighed upon you and I have supported your generous commitment to bring healing to victims, in the knowledge that in healing we too are healed and to work to ensure that such crimes will never be repeated."
The Pope also said Mass to about 25,000 people, and canonized the 18th century Spanish missionary Friar Junipero Serra despite heavy criticism.
Critics say the missionary beat and imprisoned Native Americans, forced conversions, suppressed their cultures and facilitated the spread of diseases that heavily reduced the population.
Pope Francis is to become the first pontiff to address Congress.
He'll then travel to New York to address the United Nations and visit the Ground Zero memorial to the victims of the September 11 attacks.
