Mackenzie has been with the Millers for over a decade, having first arrived in their youth setup in 2012.
He is the son of Paul Warne, former Rotherham boss and now manager of Derby County.
The 19-year-old, who signed his first professional deal with the Championship side in April 2022, featured for the Millers in last season’s Papa Johns Trophy campaign.
He could make his Seasiders debut this weekend as they welcome high-flying Cleethorpes Town to Queensgate.
Photo credit: Rotherham United
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