The Toffees announced Benitez's departure a day on from losing 2-1 to bottom side Norwich City.
Former Liverpool boss Benitez was a divisive appointment at Everton when he was drafted in to replace Carlo Ancelotti in July.
They started the season well, winning three of their first four league matches, yet injuries to key players and a dreadful loss of form has seen them slide down the table.
A win over Hull City in the FA Cup third round offered Benitez a brief reprieve, but defeat to lowly Norwich on Saturday marked a new nadir, and Everton decided to act.
Benitez cited injuries as a significant factor in their dismal form, and believed they would improve in the second half of the campaign.
"We knew it wouldn't be easy, and that it was a big challenge, both emotionally and in terms of sport," he said in a statement posted on his official website.
"My love for this city, for Merseyside and its people, made me accept this challenge, but it is only when you are inside that you realise the magnitude of the task.
"From the very first day, my staff and myself worked as we always do, with commitment and full dedication, we didn't only have to get results, but we also had to win over people's hearts.
"However, the financial situation and then the injuries that followed made things even harder. I am convinced that we would have been better once the injured players were back and with the arrival of the new signings.
"The road to success isn't easy and sadly, nowadays in football there is a search for immediate results and there is always less and less patience; unfortunately circumstances have determined the results and it won't be possible to continue this project."
Benitez left the club after a mere 200 days, giving him the third-shortest reign of any Toffees manager in the Premier League with 40 or more days in charge. Only Sam Allardyce (167) and Dave Watson (60) were at the helm for fewer days.
It was his fourth time in charge of a Premier League club and he departed with comfortably the worst win percentage ratio during his time in England.
He won just seven of his 22 games in charge, leaving him with a win ratio of 31.8 per cent – significantly behind his records at Liverpool (55.4 per cent), Chelsea (58.3) and Newcastle United (42.5).