Tries from Alex Waller and Scott Armstrong were supplemented by 14 points from the boot of Stephen Myler, with four penalties from Stephen Jones all the Welsh side had to show for their efforts, but the home pack dominated and ensured Saints' progress to the final against Leicester next Sunday.
Mallinder said: "Our pack were magnificent, they dominated the set-piece and prevented the Scarlets from playing.
"The Scarlets are a good side and if you give them a chance to play, they take it as they did in the Heineken Cup game earlier in the season. Today we prevented them from getting off to a good start and building up momentum."
Scarlets had some early opportunities to score but they failed to take them. Dominic Day was held up over the line and the try was ruled out by the TMO and a splendid run from Andy Fenby came to nothing when the wing was hauled down by an excellent cover tackle 10 metres short of the tryline.
Mallinder said: "To stop the Scarlets in that period was absolutely vital and from that moment on we were able to dominate the set-piece."
Scarlets coach Nigel Davies said: "I was disappointed with our performance. We came with a game plan but we were unable to execute it as we were far too inaccurate.
"We gave away too much ball which allowed them too many set-pieces and gave them a great platform. I was very frustrated with the first 10 minutes as we had three clear-cut chances and after that we weren't smart enough and gave away far too many penalties."
Four penalties from the impressive Myler and one from Ryan Lamb made Scarlets pay for their indiscipline and Saints were ultimately comfortable winners.
Asked about the final next weekend, Mallinder added: "I am very excited about it. It should be a great day out and hopefully it will be a sell-out. Both of us will have a few people out through international calls but they, like us, have a quality squad and it should be a cracking match."
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