Greybull is due to take over on Friday as the Mantegazzas withdraw, having run Monarch for three generations.
Jackson, former managing director of Cosmos, is among 700 redundancies to be announced when the deal completes.
He will leave on November 5 after more than two decades at the company.
The job losses are below the 900 expected following consultation, with the cost cuts demanded by Greybull achieved through some staff accepting changed terms and conditions and others moving to part-time work.
Two-thirds of the 700 job losses involve voluntary redundancies, from a workforce of 3,300. Unions representing pilots, cabin crew and engineers announced support for changes in conditions and pay cuts of “up to 30%” in a joint statement with Monarch last month.
The £75 million share-purchase deal agreed by London-based Greybull for Monarch was expected to complete on schedule, with the Civil Aviation Authority centrally involved.
One industry source dismissed reports that a £150 million pension fund deficit had proved a stumbling block.
Further high-profile departures are expected but with no immediate changes to the executive committee, which includes managing director Phil Boggon, chief commercial officer Adrian Tighe and chief operations officer Nils Christy.
Monarch showed confidence in its continued operation by revealing its summer 2015 schedule from Leeds Bradford including new routes to Alicante and Naples.
A spokeswoman told Travel Weekly: “We are on track for a solvent sale of the group to complete on Friday, which includes Cosmos holidays.”
She added: “The group will continue to offer the UK travel market a unique position as a scheduled European airline which understands the leisure market and is able to deliver allocations to Cosmos as well as third-party operators.”
Sourced from Travel Weekly