Mr Williams was appointed to the substantive position in August 2012 following the resignation of Canadian Dr Dwayne Gibbs.
This latest extension is the fourth granted by the PSC.
In a statement the PSC outlined the process used to recruit, select and appoint a Commissioner of Police.
The release also highlighted the challenges faced by the Commission and how this has impacted on the selection.
The PSC said from all the available data examined, his appraisal was rated as “good”.