Short of the first down, Lamb rolled over the top of Colts safety Rodney McLeod, put his left hand on the ground to keep his knees off the turf and sprinted to the end zone, beating Isaiah Rodgers to the edge for the Cowboys' first score.
At least two Colts defenders slowed down on the play after they thought Lamb was down, but the receiver wisely kept going without a whistle blowing.
In the fourth quarter last week, Lamb appeared to get both of his feet down on a Prescott pass, but referee Scott Novak said Lamb's heel was out of bounds.
"Man, they killed me with that one," Lamb said. "I needed that touchdown."
He got the one against the Colts with the replay official confirming the play.
For Lamb, it was his sixth touchdown of the season and fourth in the last five games, as he continues to show he is worthy of the No. 1 wide receiver spot.
The touchdown pass was the 154th of Prescott's career, passing Roger Staubach for fourth all-time in team history. He needs one more to tie Danny White for third all-time.