All areas of the business will be affected, with some staff facing up to 30% pay cuts. The company held talks with unions British Airline Pilots' Association (BAPLA), which represents its pilots, and cabin crew and engineers' union Unite.
The company revealed that 96% of pilots and 90% of cabin crew voted in favour of the move, although it did not reveal the number of employees who voted in the ballots.
The news follows confirmation that private equity firm Greybull Capital has been announced as the preferred bidder to acquire Monarch from its current principal shareholders, the Mantegazza family.
Monarch Group CEO Andrew Swaffield called the move "another step forward in our efforts to transform Monarch". He added that it "demonstrates a commitment from every member of the Monarch workforce".
Unite national officer Oliver Richardson said the union's priority is the "welfare and longer term job security of our members".
"Although the discussions over the past few weeks have been difficult and our members are sacrificing a lot, what is clear is that they remain committed to the future of Monarch and have voted accordingly," he added.
It was reported in August that Monarch plans to cut staff numbers by a third and reduce the size of its fleet. It is unclear yet how this will affect employees and whether Greybull will continue with these plans.