GMB said the figure "confirmed the worst fears" about the budget put in place by Finance Secretary John Swinney as a result of austerity measures.
Union figures show the number of workers employed by local authorities in Scotland fell from 304,300 in the first quarter of 2010 to 284,300 in the same period this year.
GMB said Glasgow City Council, the largest in Scotland, had lost some 2000 employees in that time.
A council spokesman said 1381 older staff had taken advantage of an early retirement scheme with a further 400 staff lost due to "natural attrition", such as when a worker leaves a job and is not replaced.
A further 2000 casual workers at Glasgow have now been removed from payrolls, the local authority said.
Councils across Scotland have committed to no compulsory redundancies as a result of budget cuts.
In 2010, Mr Swinney said he was facing a £1billion budget reduction from the UK Government, with public sector salaries frozen and a 3% efficiency savings target put in place.
However, Alex McLuckie, senior organiser for GMB Scotland, said: "We said he was wrong to put that budget of cuts in place and he should have used his powers to deliver a budget for growth. These latest figures confirm our worst fears."
Headcounts of staff in Scotland are down 6.6% over the period compared to 10.5% in England and Wales. Fife has lost around 1800 staff with 1600 jobs going in Highland and a further 1200 lost in Edinburgh, GMB said.
Councillor Billy Hendry, Cosla's HR Spokesman, said: "There is not enough money in the system to keep headcount at historic levels. That is why councils have been making sure their work forces are affordable and sustainable."
He said steps taken by councils to reduce headcounts, such as early retirement and voluntary severance, had been welcomed by staff and unions because jobs and services had been protected for the long term.