The 29-year-old was told in February that he was not part of Kenny Jackett's plans at the New Den - but proved his worth setting up one and scoring another on Saturday.
Millwall nearly took the lead after 10 minutes when Jay Simpson's low cross was hacked onto to the post before keeper Kieren Westwood performed heroics to deny Harris.
However the deadlock was finally broken in the 43rd minute when Harris combined with Simpson to fire the Lions in front.
Carlisle rallied after the break through Gary Madine but Millwall netted again in the 55th minute through Harris after Blues skipper Danny Livesey was sent off for hauling down the frontman.
And Carlisle's third straight defeat was confirmed in the 64th minute when Harris left Tony Craig with the simplest of finishes after latching onto Ahmet Brkovic's deep corner.