The 26-year-old, who is able to play right-back or centre-back, has found himself out of Steve Evans' plans in recent weeks and has not featured since the 6-2 defeat at Port Vale last month.
Chris Wilder's side have an injury crisis and the former Rotherham player has moved to bring in Mullins until January.
"It was essential that Johnny Mullins started to play some first-team football and with what we have available to us now, and what we might have available in the future, it allowed us to let Johnny make this move," boss Steve Evans told the club's official website.
"I have no doubt he will play a meaningful part there and we will monitor his progress with interest."
