Roselyn Akombe said the IEBC was under political “siege”, unable to reach consensus or take any decisions.
Now in the United States, she said she had feared for her safety while in Kenya after receiving numerous threats.
Last week, opposition leader Raila Odinga pulled out of the 26 October presidential run-off.
The Supreme Court annulled the result of the original 8 August poll, which saw current President Uhuru Kenyatta declared winner, after finding irregularities.
“Would it be a credible election? Absolutely not,” Ms Akombe said in an interview with the BBC’s Newsday programme.
“There is a very high likelihood that the mistakes that some of the presiding officers made during the last election will be repeated.”
Speaking from New York, she said it was not possible to make changes internally at the IEBC (Independent Electoral and Boundaries Commission).
The commission was under “siege”, Ms Akombe said, adding that it had not implemented all of the necessary measures to be able to conduct credible elections.
She said IEBC members had been voting along partisan lines, without discussing different issues on merit.
Commissioners and other IEBC personnel were facing intimidation by political actors and protesters, Ms Akombe said.
She also revealed that she herself had received a number of anonymous threats, and had been put under pressure to resign.
“I have never felt the kind of fear that I felt in my own country.”
She said she did not “feel safe enough to be able to go home” in the foreseeable future. The IEBC has so far made no public comments on the issue.
Mr Odinga, who leads the opposition – the National Super Alliance (Nasa) – said last week: “We have come to the conclusion that there is no intention on the part of the IEBC to undertake any changes to its operations and personnel.
“All indications are that the election scheduled for 26 October will be worse than the previous one.”
Mr Odinga has organised a series of protests against the IEBC in recent weeks.
Meanwhile, Mr Kenyatta says he is ready to proceed with the new vote as planned.
“We have no problem going back to elections. We are sure we will get more votes than the last time.”
“We are also telling him it is the people’s right to choose their leader. It is their sovereign right to choose their leader of choice,” Mr Kenyatta said.