Wes Thomas' first goal for the Millers put Evans' team in front after 10 minutes, a lead they held until the 76th minute when Moses Odubajo fired Orient back on level terms.
But never-say-die Rotherham shocked the O's to steal the points when Revell netted the winner in the fourth minute of injury time with a perfectly placed shot - his third goal of the week after two on Tuesday night in the victory at Tranmere.
"We have beaten an excellent team," said Millers manager Evans.
"We should have been comfortably in front at half time, but we weren't and they are a good team full of excellent players.
"In the second half the game became a little bit end-to-end and in this type of game it takes something special to win it and we were delighted to produce it for the spectators.
"We are delighted for Alex Revell with his two goals in midweek and we are delighted for him again when it's a winner in the last minute, and you could see from our supporters just how much it meant.
"We were unfortunate not to turn around 3-0 up and I warned the players at half-time to be prepared for the second half and they scored with a break away goal which perhaps we would defend better on another day.
"But we didn't give in, we were the team pressing to win and as we went into injury time we were the team throwing balls into the box and trying to win it.
"We had a go, there was some top play and our first goal was a fantastic ball in from Ben Pringle and a fantastic finish from Wes Thomas and I thought if one team was going to win it, we were. It's a fantastic result."
Source PA