Rood's double gave the 2011 champions the momentum they needed as Glenfield recovered quickly from conceding a second-minute opener to Forrest Hill's Junior Football Fern international Emma Rolston.
A Katie Jackson header in the 65th minute extended the margin to 3-1 and Glenfield needed the buffer as Rolston pulled a goal back with 20 minutes to go but, despite throwing everything at Glenfield in the final quarter, Forrest Hill couldn't find the third goal they needed to force extra time.
Substitute Casey Ridsdale came closest to levelling the game but Football Ferns goalkeeper Erin Nayler brilliantly turned over the Forrest Hill striker's goal-bound header in the dying stages.
Earlier, Forrest Hill took the lead after 70 seconds as Rolston tucked home a Briar Palmer cutback.
Rood levelled with a rasping effort from distance after stealing the ball from defender Emily Jensen.
Rood's second of the day had her side in front as she slid the ball past Forrest Hill goalkeeper Elise Donovan.
That 2-1 margin remained until the 65th minute when Jackson's header from a freekick put Glenfield 3-1 ahead but Rolston's quick reply set up a great finish but Glenfield did enough to hold on.
Coach Alexander felt his side executed their plan perfectly as they erased the memories of their stoppage time loss to Coastal Spirit in the 2013 final.
"The girls really earned that," Alexander said of the win. Forrest Hill coach Mauro Donoso was proud of his team and said Football Ferns goalkeeper Erin Nayler played a pivotal role in the outcome.
"It was two different halves really, Donoso said, "the second half was completely in our favour...
"The keeper for them played absolutely great today. I think she made the difference in the game."
The win is the Glenfield Rovers second ASB Women's Knockout Cup title in four years.
Knockout Cup
Forrest Hill Milford 2 (Emma Rolston 2', 68') Glenfield Rovers 3 (Katie Rood 6', 28', Katie Jackson 65'). Halftime 1-2.
