Aidan O’Brien celebrated a record eighth investec Derby as Serpentine led all the way to spring a 25/1 shock.
One of six runners for the top trainer, the outsider raced well clear of the pack and was kicked clear up the straight.
It soon became clear the rest of the field were playing catch-up as the maiden winner kept on well to score.
Fellow outsiders Khalifa Sat (50/1) and Amrhan Na Bhfiann (66/1), also trained by O’Brien, took second and third.
McNamara said: “I think I got a little bit of a freebie. I had huge amount of confidence in the horse having spoken to Aidan during the week.
“He said he was a horse that is going to stay a mile and six well.
“He said jump, go your own tempo and just from halfway, from the six to the five, give him a breather.
“He said he’ll keep going and he was right.
“All I could here was the horse breathing. He was in a good rhythm. He was relaxed and I couldn’t hear a thing around me.
“I didn’t want to be looking. I wasn’t sure, but I knew I was a few (lengths) clear all right.
“It’s a bit surreal. I can’t believe it.”