Helen Berger worked closely with Brand from June 2006 to February 2007, when he was a star of MTV, Channel 4 and the BBC while writing a book and performing his stand-up to sell-out crowds.
She told Southwark Crown Court that "peacock" Brand found random women online to satisfy his "insane sexual appetite" but sex often made him depressed.
In a statement to Brand's rape and sexual assault trial, Burger said she saw him "treat women poorly" and accused those close to him of "normalising bad behaviour".
She said in a statement read to Southwark Crown Court on Monday: "Though I've worked with a lot of celebrities before and seen a lot of people normalising bad behaviour, this was on another level."
She said "normalising and dismissing Russell's behaviour made me think, 'this is just the way it is here'."
Berger said Brand had "an insane sexual appetite and was sleeping with people day and night" and she recalled the comedian "getting antsy" when they were on deadlines, joking: "May I go and have sex now?".
Berger said Brand once conducted an interview with actress Sadie Frost while dressed only in his pants and an open robe.
"When I went to Russell's flat he was always in his underwear, he loved walking around in his underwear with no shirt on," saying of the encounter with Frost: "He did the whole interview in his underwear, no shirt, and an open robe."
Berger told the court she worked for Brand through his agent John Noel and would often be called on to go to the comedian's flat in Camden, north London.
"I worked a lot and was exhausted, emotional and it was a lot to deal with.
"I always had to manage my emotions around him.
"He had an insane sexual appetite and was sleeping with people day and night.
"He was very open about what his needs were, we didn't have a specific conversation about it."
Berger said part of her job was to review Brand's press coverage, including interviews where he had talked openly about his sexual encounters and desires.
"He never seemed happy after sex, more so depressed," she observed.
She told the court Brand used MySpace to find women for sex, and said: "At his house, women often came over and it was clear they were there for sex."
She remembers once finding a female celebrity asleep in Brand's bed while he was wandering around the house half-dressed, and said of the comedian: "He was like a peacock, women would always approach him."
Berger said she was instructed to get phone numbers for Brand from female fans when she first started working with him.
But she said she "felt uncomfortable" and, after seeking advice, she told Brand that she would not do that anymore.
Berger also recalled seeing Brand dismiss a woman he was chatting to at a party as soon as he realised she was in a relationship.
She said: "He turned in a way that really startled me.
"He literally switched off his charm and moved on to the next woman."
Ending her statement, Berger said she quit the job after nine months when she was "sick of him and feeling burned out".
Brand is accused of three counts of rape, three counts of sexual assault and one charge of indecent assault.
He denies all the charges.
The judge told jurors on Monday morning that the prosecution case has nearly finished and they will start to hear the defence case on Tuesday lunchtime.
The trial continues.
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