The 27-year-old's days with the Millers looked to be over when he was sent out on loan to Oxford earlier in the season after a falling out with boss Steve Evans.
But the defender made a triumphant return to the side, subsequently being handed the captain's armband and then scoring the opening goal in Saturday's 2-0 win over Aldershot which ended a six-year stay in League Two.
Mullins, who patched up his differences with Evans after a bedside chat in a hotel before an away game, wants to stay at New York for as long as possible.
"I am just chuffed we got the end result we wanted, the lads are over the moon we managed to do it. It was an unusual season, topsy-turvy," Mullins said.
"At the start of the season I thought I was surplus to requirements, I have always stated my desire to be at this club.
"For me, it is set up to be at another level up, so I just want to be part of where this football club is going. But I just want to enjoy the moment.
"(Being sent out on loan) was difficult to take because I hadn't long signed a new contract but whatever happened in the past is in the past.
"Me and the gaffer had a chat and aired our grievances but what's done is done."