George Saville's effort inside the opening minute sent the Lions on their way, before Cahill's late cameo solidified all three points.
Speaking after the match, Harris hailed Cahill's presence and ability to change the game in its critical moments.
"You turn to your bench and you've got a 38-year-old Millwall legend who's got more than 100 caps," Harris told NewsAtDen.
"He's the perfect person to bring on ... He knows what it takes. He came on and lifted the crowd.
"He put himself about and chased lost causes. I thought him, Tom (Elliott) and Fred (Onyedinma) were outstanding when they came on the pitch."
Cahill has made six appearances for the club this season and remains a key figure with Harris' men now unbeaten in their last 11 league matches.
Millwall's inspired form, also aided by recalled Socceroos defender James Meredith, has seen them rise up to 10th on the Championship standings and just six points adrift of the final playoff spot, currently held by Middlesbrough.
They next face Barnsley on Sunday (AEDT).