The famous sign performs over 14,000 revolutions every day.
Mr Mackey said it would take an investment of around £50 million to modernise the building, which is surrounded by concrete blast barriers to protect it from terrorist attack.
He said that, as the force faces staff cuts, there will also be more and more space at the site, which is ''an expensive luxury'' in central London.
''It's an expensive building to run and it's an expensive building to maintain and as we go through this change programme it's going to have space in it that we don't need," he told the Mayor's Office for Policing and Crime.
"In central London that's an expensive luxury.
"It is a 60s building, so the infrastructure and support services that are in the building, I think, from the heating and ventilation through to the IT provision, is from the 60s."
He added: "Despite the money we've spent on it over the years, it is an asset that we could use differently and better invest the money in policing."
Mr Mackey argued that reducing the number of supervisors and senior ranking officers would allow the Met to invest in more police constables on the beat.
"Core policing will still be delivered locally with more officers in neighbourhood policing," he said. "But no decisions have been made on any of these proposals."
He told the deputy mayor for policing and crime, Stephen Greenhalgh, that if significant savings were to be made then a third of the Met's property estate needed to be disposed of.
Currently, it has about 700 buildings including police stations, patrol bases and traffic garages.
John Tully, the Chairman of the Metropolitan Police Federation, said the sale of New Scotland Yard was like losing the Crown Jewels.
"It's very regrettable that it's come to this," he said.
"Clearly it's a building of age and it's got upkeep costs, but the old police authority and now Mopac have had a consistent policy of selling off property and they've now reached the Crown Jewels.
"An iconic building like New Scotland Yard is going to bite the dust.
"Thousands of people work at New Scotland Yard and it seems to me that there will be a lot of incremental costs to relocate people."
The force paid £124.5 million for the building in 2008 and it costs £11 million per year to run. Property industry sources say the building could fetch as much as £150 million.
The Met's budget plans were presented today before being considered by the London assembly with police chiefs stressing they for consultation. A final decision is expected next month.
It is expected that the move will take around two years once approvals are in place.
The Met is said to have identified a site near Whitehall, which would house between 600 and 800 staff, which is less than at its current site next to St James's Park Underground station.
The Curtis Green building on the Embankment is the former headquarters of Territorial Policing and was once the site of Canon Row police station.
The force is attempting to save more than £500 million from its annual budget of £3.6bn by 2015 in the wake of government cuts.
The five police stations that are to close are South Norwood, Richmond, Highbury Vale, Walthamstow and Willesden Green.
Met bosses are also shutting 61 counter services - some of which only receive a handful of visitors per day.
Boris Johnson, the London's mayor, promised at this years elections to maintain police officer numbers to about 32,000.
Mr Mackey added: "We want to have a greater presence in London's communities and relocate our officers in popular locations.
"Our objective is to make sure that, that underused front counters are replaced by at least three new front counters if they are closed.
"We want to end up in places that are most accessible to Londoners."