While describing it as a "serious error of judgment" that cost him his job, however, Morgan McSweeney denied having driven the appointment through by instructing officials to ignore procedures or grant clearance.
McSweeney's testimony before a parliamentary committee buttresses Starmer's account in a war of words with former foreign ministry officials over who was responsible for failings around the appointment, and whether the British premier had been told of the risks associated with appointing Mandelson.
"I advised the prime minister in support of that appointment, and I was wrong to do so," McSweeney said on Tuesday.
"What I did not do was oversee national security vetting, ask officials to ignore procedures, request that steps should be skipped, or communicate explicitly or implicitly that checks should be cleared at all costs."
McSweeney said he had made clear to Starmer there were "pros and cons" regarding the appointment but the prime minister had decided to go with who he considered to be the best fit for US President Donald Trump's administration.
The saga over Mandelson's appointment to Britain's highest diplomatic post despite his history and known ties to the late US sex offender Jeffrey Epstein has prompted calls for Starmer's resignation.
Starmer has said he was wrong to appoint Mandelson and has expressed regret but says all proper processes were followed.
He has also criticised officials for failing to tell him that a security vetting body had advised against the appointment.
Earlier on Tuesday, a former top British foreign ministry official said there was a clear sense of urgency over the timing of the appointment to make sure the new ambassador was in place "by or around" Trump's inauguration on January 20, 2025.
Philip Barton, who was the top official at the ministry from September 2020 to January 2025, said he had not been consulted on what he described as a political decision and that Starmer's office was "uninterested" in the vetting process for Mandelson to gain security clearance.
Starmer appointed Mandelson in late 2024, hailing his "unrivalled experience".
Mandelson, now 72, served as a minister when Labour was last in power more than 15 years ago.
Starmer sacked him in September 2025 after emails revealed the depth of Mandelson's ties with Epstein.
British police arrested Mandelson in February on suspicion of misconduct in public office but he has not been charged.
He does not face allegations of sexual misconduct.
The row has prompted calls for Starmer to resign for misleading parliament over the appointment.
But parliament will likely vote against a possible inquiry into Starmer over those allegations later on Tuesday.
