Harrison was given a standing ovation by some members of the Franklin’s Gardens faithful after producing a huge performance in the 15-14 success.
His 57th-minute try got Saints on the board after they headed in at half-time 14-0 down thanks to two smash-and-grab Jeff Williams tries.
And Harrison continued to push the home side forward before coming off eight minutes from time.
Mallinder’s men eventually got over again, through another impressive player, Mike Haywood and Stephen Myler kept his nerve with the match-winning conversion.
On Harrison, Mallinder said: “I’ve been saying for quite a few weeks that England should look at him (Harrison).
“He’s young, he’s keen, he’s not massive but he plays above his weight.
“His carries are immense and he can turn ball over as well.
“If England are looking for a seven, you’ve got a natural one there.”
On the game itself, Mallinder said: “It was similar to the Leicester game in conceding two tries out of the blue.
“We were attacking and then we lost two tries and were suddenly 14-0 down.
“We talked about them having a big set of forwards and tiring in the second half.
“We wanted to keep putting them under pressure and thankfully we got the results.
“I was worried all the way through from the first minute to the last.”
And on nerveless Myler, Mallinder said: “We’ve seen him time and time again kick goals like that.
“We were thinking about bringing JJ Hanrahan on to change things up and bring a freshness, but we always had the thought that Stephen was in the groove kicking wise and we’re glad we kept him on.”