GMB Scotland revealed 10,649 directly employed staff in 15 councils in Scotland were on lower wages.
This figure does not include those employed in arm's-length organisations.
The union found five councils in Scotland had introduced the Living Wage as a fully inclusive rate. It emerged some local authorities do not pay £7.20 an hour but instead supplement low hourly rates with an additional allowance.
Eight have introduced a pay supplement to the existing hourly rate, but the rest have yet to introduce the Living Wage for their low-paid workers. Some, such as Fife, have not given a commitment.
Alex McLuckie, senior organiser for GMB Scotland's Public Services Section, said: "With this number of Scottish local government workers earning less than £7.20 per hour, low pay is not a thing of the past in local government."
From October this year, the National Minimum Wage for workers aged 21 and over will be £6.19.
A spokesman for COSLA, the local authority umbrella body, said: "Councils are taking very seriously the circumstances of those who are at the lower end of the pay scales. Councils have responded with a Living Wage of £7.20 per hour and will maintain dialogue with trade unions."