McCabe spokeswoman Melissa Schwartz said that his GoFundMe campaign would stop accepting donations Monday night after more than tripling the original goal of $150,000.
"The outpouring of support on GoFundMe has been simply overwhelming and has led to contributions that have left us stunned and extraordinarily grateful," Mr McCabe said in a statement.
"The fact is that if I am going to continue taking a stand against the unfair way I have been treated, I will need the help of a talented and courageous team behind me," he said.
"Hopefully our efforts, fueled by this incredible support, will encourage others to stand up for themselves, and the truth, as well."
After barely a week, the online fund had taken in $541,646 around midday Monday.
Mr McCabe was fired by Attorney General Jeff Sessions on March 16, 26 hours before he was planning to retire, based on allegations he had lied in an internal probe over his handling of the 2016 investigation of Democratic presidential candidate Hillary Clinton.
The firing came after nearly a year of attacks against him by Trump, who hailed his dismissal as "a great day for the hard working men and women of the FBI" and "a great day for Democracy."
"He knew all about the lies and corruption going on at the highest levels of the FBI!" Mr Trump tweeted at the time.
Mr McCabe says he is the victim of a Trump administration "war" against both the FBI, and the special counsel probing allegations of Russian election meddling. The final report by the Justice Department inspector general on the Clinton case has yet to be released.
Mr McCabe said he raised the money to pay for legal support in dealing with ongoing Justice Department probes, testifying at congressional inquiries, and possible lawsuits he could file.
He has recruited a former Justice Department inspector general, Michael Bromwich, to lead his legal support team.