Irish striker Sammon had spent the first half of the season with the Tractor Boys, but he came back to haunt them with Rotherham's second goal to add to Matt Derbyshire's first-half opener.
Ipswich lost ground on the Sky Bet Championship automatic promotion pack as Rotherham earned back-to-back home wins for the first time since September.
The visitors should have gone ahead with the first play of the game when Daryl Murphy got in around the back of the defence, but he hit his shot tamely at Adam Collin. Murphy then fired over the bar when the ball dropped kindly to him from a corner.
A cynical foul from Cole Skuse on Paul Green earned him a booking and Rotherham a free-kick that presented them with their first chance. Ben Pringle's delivery was met by Kari Arnason but his header took a deflection on its way through to Ipswich goalkeeper Bartosz Bialkowski.
Neither side managed to stamp their authority on the match in the opening half hour, with only David McGoldrick forcing a save from distance.
Two more shots from long range followed, with Paul Anderson ambitiously having a go from 30 yards and Jay Tabb dragging a volley wide from the angle of the box.
Rotherham then went ahead in the 38th minute, with Derbyshire burying a diving header from close range after captain Craig Morgan had headed the ball back across the six-yard box following a Pringle free-kick.
The home side began the second period on top and Conor Newton flashed a shot wide from the edge of the box. Freddie Sears and Teddy Bishop were brought on by Ipswich manager Mick McCarthy in an attempt to turn the game around.
Ipswich almost immediately got on terms when Tommy Smith flicked Skuse's free-kick goalwards and forced Collin into an acrobatic save.
They kept the pressure up and Sears had his first effort on goal after controlling Tyrone Mings' shot and drilling an effort which Collin had to turn behind for a corner.
Rotherham loanee Zeki Fryers then cleared Murphy's header off the line as the away side stepped up their pursuit of an equalising goal.
They were to be hit at the other end by their former frontman Sammon, who blasted in on 64 minutes following a corner that could not be cleared properly.
Morgan had to be alert to the danger twice at the other end when Sears looked to sneak in on goal in response.
Substitute Jordan Bowery then missed a late one-on-one but it did not matter for the home side.