Southend ended Rotherham's unbeaten home record in style and extended their own winning run to three games.
Peter Gilbert opened the scoring when he lashed home from the edge of the box after the Millers' defence had failed to clear from a corner.
On-loan midfielder Michael Timlin doubled their lead in the second half with a powerful left-footed volley and made it 3-0 with an 18-yard strike.
Striker Neil Harris completed the drubbing with a tap-in.
VIEWS FROM THE DRESSING ROOM
Rotherham boss Andy Scott told BBC Radio Sheffield:
"It was totally unacceptable from start to finish. They were better then us in every department.
"My team was out-worked and out-fought and I'm very disappointed and upset about it. The players are well aware of that fact.
"One thing I'm going to have to take on board is that the players can't take the information on or carry out what's being asked of them. If that's going to be the case than the players won't be playing in the side.
"All we, the fans and chairman wants is to see their team working hard and putting everything they can into it."
Southend Manager Paul Sturrock told BBC Essex:
"Obviously, we are very, very pleased, (Liam Dickinson) set the tone from the first minute.
"He just seemed to be a catalyst to everybody rolling their sleeves up and giving us 100 per cent.
"The important thing for us was that the shape of the team was exactly the way we wanted it, it was planned that way."