Northampton coach The home side only led 12-0 after 60 minutes through four Stephen Myler penalties, but then blew the French side away with five tries in the final quarter to avenge last week's defeat in the reverse fixture.
Mallinder said: "I'm pleased with our performance. I was disappointed last week, we attacked well but our discipline and defence was poor.
"At half-time I told them not to get frustrated and we needed to be a little more accurate."
Mallinder introduced key players Lee Dickson and Sione Tonga'uiha on the hour and they made important contributions in breaking down the Castres rearguard.
Mallinder added: "The subs made a considerable impact today and all credit to them as we needed to keep their pack moving around."
Northampton have no hope of progressing further in the Heineken Cup, but still face games against Munster and Scarlets in January.
"That win gives us a chance to make it into the Amlin Challenge Cup but we will need a couple of good wins against those tough sides to have a chance of getting there," Mallinder added.
"We are focusing on our next three league games against Bath (home), Newcastle (away) and Quins (home), and are aiming for three victories."
Castres chose to rest a number of last week's victorious side and paid a heavy penalty.
Castres coach Laurent Travers said: "Northampton had points to prove. They conceded 40 points at ours last week and are not Heineken Cup finalists for nothing.
"We defended well for an hour but, once their first try went in we had no answers and it was just too difficult."
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