They shed new light on the contentious decision and heap more embarrassment on beleaguered Prime Minister Keir Starmer.
Mandelson was fired after nine months, and fallout from the misjudged appointment has left Starmer fighting for his job.
In a handwritten note to then-foreign minister David Lammy on November 18, 2024, Mandelson writes: "I just wanted you to know that if you were minded to appoint me (as ambassador), I would make sure you never regret it."
"I fear that navigating Britain's interests through the Trump administration will require super-human skills and luck and a massive team effort," Mandelson said.
His appointment was announced the following month.
A first trove of files published in March revealed ministers had been warned that Mandelson's friendship with Epstein exposed the government to "reputational risk".
It was later disclosed that Mandelson had been approved for the ambassador's job despite failing security checks, a revelation that sparked bitter blame-trading between Starmer and senior civil servants who oversaw the security vetting.
The files released on Monday shed some light on discussions within government about the appointment but key details remain a mystery.
Some documents have been redacted on national security grounds, and others have been withheld at the request of police investigating Mandelson for alleged misconduct in public office.
It remains unclear why Mandelson failed security checks.
The summary of his vetting process was not among the documents released as it is part of the police investigation.
Officials have said it is not because of his ties to Epstein.
A background report drawn up by officials before Mandelson was appointed, released by the government in March, flagged business ties to Russia and China as a concern and noted his resignations from two previous Labour governments because of scandals over money and ethics.
Also missing is any record of what measures, if any, were taken to mitigate the risk of giving Mandelson the job.
Mandelson refused to hand over information from his personal phone to officials, and the government "has no further recourse to search the personal devices of Peter Mandelson," the documents released on Monday say.
Mandelson, 72, was briefly arrested in February by detectives investigating allegations he passed sensitive government information to Epstein when he was a minister in the United Kingdom government a decade and a half ago.
He has been released without bail conditions as the police investigation continues.
Starmer fired Mandelson in September 2025 after an earlier release of documents showed he had maintained contact with Epstein after the financier's 2008 conviction for sexual offences involving a minor.
Critics say Starmer's decision to appoint Mandelson is evidence of bad judgment by a prime minister who has made repeated missteps since he led the Labour party to a landslide election victory in July 2024.
The documents include emails and text messages between Mandelson and government ministers and advisers, some discussing weighty affairs of state, others sharing gossip or unusual details.
At one point, Mandelson and other officials discussed commissioning a red dispatch box like those used by UK government ministers, inscribed "President of the United States" as a gift for Trump during his September 2025 state visit to the UK.
But the box was delayed by political concerns and issues at the manufacturer.
Mandelson expressed his frustration with the hold-up, saying officials had known about the request for months.
"And nobody had the wit to say anything," Mandelson wrote to Morgan McSweeney, then Starmer's chief of staff, on August 27.
"What incompetence."
with Reuters
