Mitchell was named man of the match by BT Sport after yet another electric showing in the bonus-point success.
It ensured Saints would finish the weekend sitting fourth in the Gallagher Premiership with two games of the regular season remaining.
Saints are set to complete the campaign with a trip to Saracens before welcoming Newcastle Falcons to the Gardens on the final day.
And when asked about the significance of the success against Quins, Mitchell said: "It's massive.
"We knew we had three games left and that if we won all three we'd be in the top four.
"That's one out of three!
"We're pretty confident now.
"We've got some momentum behind us now and we feel we can get that top-four spot."
Saints, who led 26-17 at half-time, were beset by injuries in the clash with Quins, losing six players at various times.
But they stayed strong and stuck it out, securing the victory thanks to James Grayson's nerveless long-range penalty.
"We were pretty content at half-time because we were very good in attack, we kept on moving the ball and we were scoring tries," Mitchell said.
"Defensively we were a bit poor, missing tackles, a few system errors.
"It was a great game for the neutral though we were a bit frustrated at half-time but also happy.
"We're not sure what went wrong at the start of the second half, whether that was a lack of energy or whatever.
"We know that Quins are such a top outfit and they were going to come back firing so it's something to work on for a few weeks' time to get better at."