Paul Charles Wilkins, 70, of Littleport, Cambridgeshire, travelled to California in January to "engage in illicit sexual conduct" with boys aged 10 and 12, US officials said.
He initially denied four charges but later admitted transporting the images.
Under a plea agreement he is expected to be sentenced to 13 years in January.
He was jailed for 56 months in the UK in 2011 for possessing indecent images of children.
More on this and other news from Cambridgeshire
Wilkins, who holds dual UK and US citizenship, was on probation when he left the UK on 31 January.
He set up an online site enabling him to arrange to meet boys, but was caught in a sting operation in Palm Springs, California, by undercover Homeland Security Department special agents.
He had paid one agent $250 (Â£170) to have sex with a nine-year-old Mexican boy.
Wilkins had initially denied four charges of travelling with the intent to engage in illicit sexual conduct, attempted sex trafficking of children, transportation of child pornography and possession of child pornography, the US District Attorney's Office said.
A trial date was set for November, however, at a hearing on Wednesday he pleaded guilty to transporting child pornography under a plea agreement with the US Attorney's Office.
He agreed to a 13-year prison term and a lifetime of supervised release, the US Department of Homeland Security said.
Wilkins also admitted in court to travelling to the US in order to have sex with two young brothers, paying to have sex with the nine-year-old, possessing pornography and bringing a computer into the US containing "graphic images of boys ranging in age from five to eight".
US Attorney Eileen Decker, chief federal law enforcement officer in the Central District of California, described Wilkins as a "predator" and "extremely dangerous".
The plea agreement meant he would "be in prison for well over a decade," she said.
As part of the agreement the three other counts are expected to be dismissed when Wilkins is sentenced for the transportation charge.
Wilkins was detained as part of Homeland Security's Operation Predator, an international initiative to protect children from sexual predators.

The 24-year-old, who turned down the offer of a new Crewe deal, becomes Walsall's second summer signing, following the arrival of fellow 24-year-old Luke Leahy from Falkirk.
"Jon has got plenty of experience of League One from his time with Crewe," said Walsall manager Jon Whitney.
"We'll be getting Jon at a good stage of his career."
Find all the latest football transfers on our dedicated page.

It was taken by ex-students and staff from UWC Atlantic College in the Vale of Glamorgan who also stayed to help.
The boat will be used to try to ensure the safe landing of boats from Turkey.
A narrow stretch of water has become a crossing point for people from Syria and other countries which has claimed many lives.
College alumni Robin Jenkins and Lily Eckersley-Jones helped transfer the rescue boat to Lesbos with college tutor Michel Creber where another former student, Dr Annegret Berne, had been volunteering.
Mr Jenkins, originally from Llantwit Major and a lecturer at University of the Arts London, said: "The faces of the people on board when we come alongside is something that I cannot put in to words.
"We were a reassuring sign of safety.
"We must do more and we will start on constructing another boat."
The first vessel, named LB1 Red Mist, had formerly been used by the college at St Donat's, near Llantwit Major, and its students before being sent to Lesbos.
It is now being used by a group called the Lifeguard Hellas Training Centre.
Its design is the same as the original rigid hull inflatable boat (RIB) made in the early 1960s by students from the college and which has become the model for RNLI inshore boats across Britain - known as the Atlantic class.
Mr Jenkins set up a help group called the Atlantic Pacific Rescue Boat Project to deliver a boat to Japan to assist lifeguards and now others are being made to help rescuers in Lesbos.
He said college students working with tutor Michel Creber have already began work on a second RIB and he had also received a donation on Tuesday to cover the cost of making a third.
A convicted British sex offender caught trying to pay US undercover officers for sex with a boy has admitted taking pornographic images into the country.
[NEXT_CONCEPT]
League One club Walsall have signed Crewe Alexandra defender Jon Guthrie on a two-year contract.
[NEXT_CONCEPT]
A Welsh lifeboat has been sent to Lesbos to help people trying to ensure the safety of refugees and migrants arriving by boats on the Greek island.