Katie Perrior, who served as director of communications for 10 months, said she and others were not able to "speak freely" and her advice was not heeded.
She accused the PM's chiefs of staff Fiona Hill and Nick Timothy of treating ministers with a lack of respect.
The PM is being urged to make changes after losing her majority.
The BBC understands Mrs May has been warned she faces a leadership challenge on Monday unless she sacks her two closest advisers.
Mrs May, who is seeking to form a government with the support of the DUP, has said she will look at "personnel issues" amid anger in Tory circles about the role of Mr Timothy and Ms Hill in the drafting of the party's manifesto and the overall conduct of the election campaign.
Ms Perrior said Mrs May was a "good person" who would take personal responsibility for the campaign but suggested the set-up in No 10 was not working.
"Being in the Home Office for such a long time with her top team, she became accustomed to that being it and of course running the Home Office is very different from running the country," she told BBC Radio 4's Today.
"She needed to broaden her circle of advisers and have a few grey hairs in there who had been around the block a bit in politics who could say 'don't do that and don't make enemies when you don't need to'."
She suggested that Mr Timothy and Ms Hill - the PM's "gatekeepers" who worked for her in the Home Office - were too protective of the prime minister and had a negative influence on how No 10 functioned, often "bellowing" at other staff.
"We would be able to speak freely if they weren't around and if they were, you don't speak," she said.
"Sometimes we would sit there and hear Fiona come up with things which, quite frankly, were crazy and say nothing."
Asked if the two aides treated cabinet ministers badly, as has been reported in the newspapers, she replied "There was not enough respect shown to people who had spent 20 years in office. I felt sending people rude text messages was unacceptable.
"What the PM needs when you are going through a tough time like negotiating Brexit is diplomats, not street fighters. They only know one way to operate and that is to have enemies. I am sure I am one of these this morning."
Ms Perrior, a PR specialist who previously worked on Boris Johnson's 2008 mayoral campaign, said she had expressed her concerns before leaving in April about how things were done and "that did not go down very well, as you can imagine".
Although she had never intended to stay in the role for more than two years, she said she concluded that remaining was "pointless" as former journalist Ms Hill had made it "pretty clear" she wanted to be communications director during the campaign.
"If you feel you are actually not being listened to then there is not point you being there... in communications, you don't have a dog and bark yourself. If you don't want to hire someone for their advice, you should not be hiring them."
She suggested the PM could have been better advised during the campaign, in which she declined to take part in the leaders debates, and also in its aftermath.
The PM made a defiant statement outside Downing Street before giving an interview hours later in which expressed regret for the Conservative candidates who had lost and promised to "reflect" on the result.
It has been reported that the second statement came after being told to do so by senior MPs.
Ms Perrior added: "If you are not this great communicator who is on to the phone this morning telling candidates 'I am sorry or I want to work with you in future, come with me' then you hire people who do that for you."
"I am afraid she did not have those qualities herself but she did not hire people who had them either."
Neil Carmichael, one of the Tories to lose their seats at the election, said there needed to be a "thorough shake-up" at Downing Street.
"You can't really run a government from an ivory tower situation," he told BBC News. "I do think Theresa May requires a more broadly based set of advisers."
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The atmosphere in Downing Street under Theresa May is "pretty toxic" and its operation "dysfunctional", a former aide of the prime minister has said.