Mills is on a season-long loan at Middlesbrough and the Teessiders have so far failed to turn his move into a permanent situation.
The England international full back still has a strong affection for Leeds and says he would have no problem playing for United in Division One.
"We had ups and downs at Elland Road," he told Sky Sports News. "Both good and bad, especially in Europe. Leeds will always be very close to me and I hope that they stay up.
"I am still a Leeds player and if I have to go back there, whether it be in The Premiership or not, I am happy to go back and play for Leeds. If it transpires that is not the case, we all know they may have to sell players and if I am one of them, then so be it.
"But at the moment I am due to go back to Leeds on May 16 and become a Leeds United player again."
Mills helped the Riverside club win the Carling Cup, and he admits he has enjoyed his spell at Boro.
"I was not going to play any football while Peter Reid was at Elland Road," he added. "I watched the first game against Newcastle from the stands and was down. Certain things happened while I was away with England and I moved to Middlesbrough.
"I have had a good season, the fans have seen this and I have enjoyed myself."