New York Mayor Michael Bloomberg joined Cameron and his wife, Samantha, at the memorial to victims of the Sept. 11 terror attacks.
The Camerons touched the bronze plaques inscribed with victims' names and gazed into the pools of water where the twin towers stood.
They also were joined by Charles Wolf, whose Welsh-born wife Katherine died at the trade center.
Cameron also held a questions-and-answer session with students at New York University and visited the New York Stock Exchange. His flight back to England was scheduled for late Thursday.
At least 67 citizens of the United Kingdom died at the trade center on Sept. 11, 2001.
Wolf, whose wife had started a job with professional services firm Marsh & McLennan Companies Inc. at the trade center just three weeks earlier when she was killed, said it was an honor to meet Cameron.
"It was very, very nice for both he and his wife to come," Wolf said after their meeting. "It was very personal as well as being official."
Wolf referred to Cameron's son Ivan, who had cerebral palsy and severe epilepsy and died in 2009 at age 6. "Of course they have had their own loss of their child, and when you are with someone who's had a recent loss like that, you know you are with people who get it, who care," Wolf said.
Before visiting the memorial, the Camerons toured One World Trade Center, the skyscraper under construction at the site. They admired the view from the 21st floor.
At New York University, Cameron took questions from college students and faculty about everything from the Syrian conflict to Scottish independence.
He highlighted the "special relationship" that the U.S. and Britain enjoy during his brief opening remarks Thursday during a questions-and-answer session at the school's Manhattan campus. He said his country should increase trade with its traditional partners like the U.S.
Earlier Thursday, Cameron visited businesses in New Jersey that have gotten a boost from local government aid.
With Newark, N.J. Mayor Cory Booker as his guide, Cameron stopped by businesses that have been helped by local economic development aid. They included a restaurant decorated for St. Patrick's Day and a coffee shop where the two men grabbed a table and talked.
During a walking tour of downtown Newark, Booker said the prime minister was interested in learning about local economic development initiatives as well as how local governments are structured.
Cameron met earlier with the mayor - a Rhodes scholar who got a degree from Oxford in 1994 - at City Hall to discuss education reforms in New Jersey's largest city and other initiatives. Cameron expressed interest in Newark's charter schools, public safety and police reforms, and programs to help former prisoners re-enter society.
"I'm a great believer that big-city leaders can do great things for their communities, so it's great to have this session where we can quiz you on some of the projects you are running and exchange some information on schooling, on crime, on police accountability, on regeneration - those are the things we're interested in hearing about but it's great to be so welcome," Cameron said during a sit-down with Booker and his department heads at City Hall.
Booker, who told Cameron about his student days in England, said he spent two formative years studying there.
"It really was a time for me that fostered some of my boldest dreams about what we could be doing back in the United States," Booker told Cameron as they sat around a conference table at Newark City Hall. "This partnership is very special to me, and I'm hoping that some of the things we've learned, some of the lessons we've learned the hard way, hopefully they can make a small contribution to the things that are going on in England."
Cameron arrived in the U.S. on Tuesday. His trip included a college basketball game with President Barack Obama and a state dinner at the White House.
The two leaders discussed the war in Afghanistan at a joint appearance on Wednesday. They said that NATO forces would hand over the lead combat role to Afghanistan forces next year as the U.S. and its allies aim to get out by the end of 2014.
The announcement came after the war effort was rocked by the burnings of Qurans at a U.S. base, deadly protests against Americans and a shooting rampage by a man believed to be a U.S. soldier that left 16 Afghan civilians dead.
Obama, 50, and Cameron, 45, appeared to enjoy each other's company, and they exchanged gifts. The Obamas gave the Camerons a wood- and charcoal-burning grill engraved with American and British friendship flags, complete with his-and-her White House chef jackets embroidered with the Camerons' names.
First lady Michelle Obama also gave Samantha Cameron a crystal vase of White House honey. And the Cameron children got bean bag chairs embroidered with their names and the presidential seal.
The Camerons reciprocated by giving the Obamas a pingpong table.
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