Paul Holmes, an assistant to Home Secretary Priti Patel, said he was “shocked and angered” by the revelations in the report.
He added his work for constituents had been “tarnished by the toxic culture that seemed to have permeated No 10.”
Ms Gray’s report highlighted widespread Covid rule-breaking and drunkenness.
In her report, senior civil servant Sue Gray said many events held during Covid restrictions “should not have been allowed” and the prime minister and his officials “must bear responsibility for this culture”.
It also found “multiple examples of a lack of respect and poor treatment” of security staff and cleaners by officials when gatherings took place.
Mr Holmes said the findings over treatment of staff were “disappointing and unacceptable”.
He wrote that they showed a “culture in No 10 that was distasteful” – but added he was glad that Prime Minister Boris Johnson had apologised over the treatment of staff.
“It is clear to me that a deep mistrust in both the government and the Conservative Party has been created by these events,” he added.
He added his “distress” at the report’s conclusions had led him to conclude he wanted to “focus solely” on being an MP.