Starmer's allies have sought to play down the importance of the first set of documents, saying an ongoing police investigation meant some of the more revealing exchanges were being withheld.
Further documents will be released at a later date, papers which the UK prime minister's team say will prove Mandelson lied to Starmer before his appointment to become the United Kingdom's top diplomat over the extent of his relationship with Epstein.
Starmer decided to sack Mandelson after emails released by Bloomberg "revealed a depth and extent of a relationship with Epstein which he had not been aware of previously when he made the decision to appoint Mandelson", according to a No 10 private office note released on Wednesday.
The memo said that at a meeting on September 11, the prime minister "referenced his concern at the judgments and views expressed in the cache of emails released by Bloomberg, that the answers Mandelson provided to FCDO PUS (Foreign, Commonwealth and Development Office permanent under-secretary) Olly Robbins regarding the emails did not give him confidence that there were not further revelations to come and the serious discomfort in the PLP".
"The emails revealed a depth and extent of a relationship with Epstein which he had not been aware of previously when he made the decision to appoint Mandelson.
"On this basis, he proposed to ask Mandelson to resign from his post as HMA Washington."
Mandelson, a government minister when Labour was previously in power more than 15 years ago, quit the House of Lords upper house of parliament in February over his links to Epstein.
He was arrested last month for alleged misconduct in office.
Files released by the US Justice Department in January included emails suggesting Mandelson had leaked government documents to Epstein, and that the convicted offender had recorded payments to Mandelson or his then-partner, now husband.
Mandelson has said he does not recall having received payments.
He has not commented publicly on allegations he leaked documents, and did not respond to messages seeking comment.
with PA