Sam Baldock's early strike looked like being the difference between the two sides for much of the game, but Joe Mattock's late bundled effort earned Rotherham a point.
Baldock headed the first chance of the game over from Andy Yiadom's cross.
He was more clinical in the ninth minute when he sprung the offside trap from Josh Sims' pass and tucked the ball past goalkeeper Marek Rodak and into the bottom corner.
Reading almost added to their lead when John Swift went on a solo run before dragging his shot wide from the edge of the box.
The ball then fell kindly to Richie Towell but his placed shot was easily cleared away from danger by Yiadom.
Will Vaulks was the next to have a go from outside the area but his effort did not trouble Anssi Jaakkola.
The visiting keeper's first save did come from Vaulks' tame effort while Swift's liveliness almost caught out Rotherham again but Rodak pulled off a good save.
Garath McCleary had a great chance to send Reading in two goals ahead at the break but he held off his run and ended up shooting straight at Rodak when clean through on goal.
Rotherham manager Paul Warne introduced striker Jamie Proctor for the start of the second half and his side immediately started playing with more purpose.
Vaulks had an effort blocked by Tyler Blackett and Towell forced a save out of Jaakkola.
Reading were denied a second by the woodwork when Sims' close-range strike from Swift's pass struck a post and bounced away to safety.
Proctor then showed skill and strength on the edge of the box before his effort was kept out by Jaakkola.
Vaulks should have done better when he lifted an effort over the top after the ball reached him at the back post from a corner.
Reading were still causing trouble of their own and skipper Leandro Bacuna came agonisingly close to sealing the points with a curling strike from the edge of the box.
The Millers pushed for one of their trademark late goals but found Reading in resilient mood with Yiadom and Chris Gunter making goal-saving challenges.
The crucial goal finally came in the 90th minute with Mattock poking the ball in from Joe Newell's corner.
An injury-time winner almost came from Vaulks but his volley dribbled just wide of goal.