But less than a month later he has been sold to Oxford for an undisclosed fee, signing a three-year deal, after being told he was not going to be manager Steve Evans' first choice.
The defender, who spent time on loan with the U's last season, wore the armband as the Millers won their last five games to seal a second-placed finish in League Two and scored the goal that put them on their way to clinching promotion against Aldershot.
However, things changed when he returned from his summer holiday, with Evans preferring new recruit Richard Brindley at right-back and Claude Davis, Craig Morgan and Kari Arnason at centre-back.
When Oxford came in for him, the 27-year-old had no hesitation in joining, though was left heart-broken by leaving the club where he was the longest-serving player.
"It happened really quickly, I found out at the back end of last week that the club had accepted an offer," he said. "That made my mind up. I realised I was down the pecking order.
"It wasn't very nice after how last year ended, but that's football. I got in my head that I would be captaining the side in League One after what happened last year, but it wasn't to be.
"I felt early on from pre-season that he was going the other way. I decided pretty quickly after that to move on and I'm gutted.
"What happened last season was great to be a part of and I wanted to come back prove myself at the higher level, I wasn't given the opportunity to do that."
Evans has said in the wake of Mullins' exit that the security of a three-year deal was a deciding factor, while accusations have been levelled at him that he was unhappy travelling from his home in the midlands.
"If I was going to play at Rotherham, I'd still be there. That's it," he added. "All the talk of the travelling and security is highly annoying.
"I've got a place in Nottinghamshire so that wasn't an issue, it was purely football reasons.
"The manager said he rated Claude Davis, Craig Morgan and Kari Arnason higher than me and I can't have any qualms about that. Football is about opinions and I can't hold a grudge against anyone for having an opinion."