The 22-year-old moved to Pride Park on loan in February, just two months after he signed for the Lions, as he could not settle in south London.
Robinson hit three goals in eight starts for Millwall and has scored twice in 13 appearances for County.
Meanwhile, boss Kenny Jackett has agreed the signing of rookie West Brom goalkeeper Ryan Allsop.
The England under-17 international has undergone a medical at The Den and will put pen to paper on the move on July 1.
Allsop, who will turn 19 next month, has yet to play a senior game but was on the bench for the Baggies' Barclays Premier League clash with Wigan in May 2009 after an injury to Scott Carson.