The 22-year-old from Forfar, partnered by team-mate Adam Balon in the number 72 Lamborghini Huracan GT3 Evo, looked on as the Englishman hounded the leader over the final 28 minutes of the one-hour race.
Sadly the team came up short by the narrowest of margins and at the chequered flag the gap was just 0.340secs.
“Adam did a fantastic job throughout his stint,” Mitchell, who set the fastest lap of the race, 1min 47.641secs round the 2.96-mile Norfolk circuit, said.
“I made a good start from fifth on the grid.
"One car went off on to the grass, then I was immediately able to overtake one of the Mercedes at turn one. A few laps later I took another Mercedes when he made a small mistake after I put him under pressure.
“That put me up to second and I was able to stretch the Lamborghini’s legs and haul in the leader. I was really pushing and set the fastest lap of the race in the process, which was great and really emphasised how quick the car is.
“The main thing was to stretch the gap to the cars behind, which meant it became a two-horse race in the second half between us and the RAM Mercedes.”
With 28 minutes of the race remaining, Mitchell pitted and handed the car to Balon.
Rejoining still in secomd, he set about closing the gap to the leading Mercedes, gradually closing the gap from 3.105s to 0.510s with six laps remaining.
“Despite everything Adam threw at the Mercedes, he just couldn’t get past,” Mitchell added.
“Adam’s pace throughout the second stint was fantastic. He really hounded the Mercedes.”
And Mitchell, who along with Balon battled through from 10th on the grid in race one to finish sixth, was delighted with the weekend’s performance which saw the duo move up to fourth in the title race. Just 2.5pts separate fourth from second in the championship.
“I feel we needed the results in these two races,” he added.