In the 14th minute, Rotherham conceded possession on the halfway line to give Downing a clear run at goal. He surged into the penalty area before striking the ball across goal into the back of the net.
De Pena nearly doubled the lead three minutes later but his header hit the bar from Nugent's cross.
Both teams struggled to find their shooting boots in the second half, as Rawson and Andreu again shot off target.
De Pena, Adam Clayton and Nugent also failed to test Camp but Leadbitter called the veteran goalkeeper into action in the 74th minute.
His volley was directed towards the top left corner but Camp dived in front of the ball to keep the score at 1-0.
Friend nearly gifted Rotherham an equaliser when he gave the ball away to Jordan Bowery. The striker was through on goal but missed the target from 10 yards.
The Boro defender nearly made up for his error in the 83rd minute but his shot from the edge of the area struck the left post.