"Chartered Institute of Public Finance and Accountancy (CIPFA) figures reveal full-time paid staff fell from 1,112 in 2012-13 to 890 in 2016-17 - a 20% drop.
Meanwhile, the number of volunteers increased 13-fold, from 93 to 1,288.
The Welsh Local Government Association (WLGA) said councils were finding "innovative ways" of keeping services in a climate of cuts.
CIPFA chief executive Rob Whiteman said the overall budget cut to Welsh libraries in the last decade had been less than that in England - 18% compared to 25%."
