The 27-year-old scored in injury time to put the finishing touches on a 2-0 win over Aldershot which also secured the Millers promotion and ended their six-year stay in League Two.
Frecklington, who joined in January from Peterborough, goes alongside club greats Alan Lee, Rodney Fern and Tony Cunningham in scoring goals that have won the club promotion, and he is delighed.
"It is probably one of my most important goals. It's a goal that has helped Rotherham to get promoted," he said. "It's a great feeling, I dont think there is a better feeling in football is there? Winning promotion at a club like this, everyone is delighted as you could see."
It looked as if the heavily-backed Millers would be consigned to the play-offs after an inconsistent season, but they came good at the end with five successive wins and Frecklington was happy to reflect on a job well done.
"We did what we needed to do," he added.
"We made it difficult for ourselves, some of performances this season we have let ourselves down but we managed to come good at the end. I don't think anyone would have predicted five wins in a row to finish it off.
"Everyone has got a sense of relief that we have managed to achieve what we set out to do at the start of the season."