Sedgwick moved to Deepdale in November for £280,000 but will earn his old club £50,000 if his new side reach the play-offs and £100,000 if they are promoted.
Preston stretched there impressive record to one defeat in the last nineteen with a sparkling second-half come back.
The visitors made a nervy start but striker Richard Cresswell came close to grabbing an opener with a clever spin and shot from the edge of the box that flew just over.
Sedgwick found Cresswell with a super throughball soon afterwards but the striker was again off target.
But it was Rotherham, already contemplating life in the lower division, who grabbed the lead, silencing Preston's impressive 2,200 following.
England Under-19 striker Will Hoskins drilled a cross from the left but sliced it and the ball swung wickedly to beat Carlo Nash and drop in at the near post.
The shock sparked Preston into life and a surging run from Cresswell forced Rob Scott to cut down the ex-Sheffield Wednesday frontman inches outside the penalty area.
But in the 51st minute it was Sedgwick who struck with his third goal since switching to Preston.
David Nugent pulled back an astute ball and the winger raced in to place an accurate side-foot shot wide of Mike Pollitt from 20 yards.
Sedgwick, who spent ten years at Millmoor, earned a respectful round of applause from the home fans for the goal - and his low key celebration.
Preston clinched the points after 64 minutes. A free-kick from Eddie Lewis was headed on by Nugent and Youl Mawene ghosted in unmarked and shot past Pollitt from eight yards.