Fresh off two straight losses and with questions asked of their attack in last week's defeat to Canberra, it was the Sharks who offered more with the ball in Perth.
After withstanding waves of attack in the first 35 minutes without buckling, Cronulla broke the Sea Eagles' backs before halftime.
The Sharks then led 14-6 and 24-12 in the second half, before two long-range intercepts thrown by Braydon Trindall let Manly back into the game on both occasions.
But the Sharks were ultimately able to hold on, leaving both sides with a 3-3 record after six rounds.
Fullback Will Kennedy was superb in the win, Trindall set up two tries, while Nicho Hynes and Blayke Brailey both pulled the right rein at key moments.
"They were obviously in a lot of ways a lot better," Fitzgiboon said of Trindall and Hynes.
"They were better in the way they combined and kicked. A lot of improvements on the details of their plays.
"And then there were a couple of moments they would like back. That is part of the add-on and building combination.
"Nicho was on the ball, controlling, orchestrating and calling the shots. He steered the team around and ran the ball when he had to run the ball."
While Manly were unable to make use of their ball and missed their main man Tom Trbojevic's spark in attack, Cronulla produced in key moments.
With WA premier Roger Cook watching on at Optus Stadium and Perth's hopes of a franchise hanging in the balance, it was Trindall who broke the game open.
He first sent winger Sam Stonestreet over with a long cut-out ball, before grubber-kicking for the winger's second on halftime.
The second half was all Kennedy.
He was denied a 90-metre effort from a kick return when Stonestreet was ruled to have provided an escort run on the chase.
But the fullback wouldn't be stopped later on, benefiting from a Jesse Ramien offload when he stepped, dummied and beat five defenders to the line.
Kennedy had earlier also produced a key moment in defence, denying a Manly try when he stuck out his foot to trap a grubber kick after a Sea Eagles break.
Then when Manly got back to 14-12 courtesy of a 90-metre Luke Brooks intercept, it was Hynes and Trindall who slowed down the comeback.
Linking up on the left edge, Hynes produced a double-pump pass to catch out the defence and give Ronaldo Mulitalo the space to send Mawene Hiroti over.
Brailey also had a smart play late, putting Tom Hazelton over for a try with a short ball from dummy-half when Manly expected a field goal attempt at 18-12.
That try ultimately proved crucial, ensuring the Sharks had enough of a bubble when a late Tommy Talau intercept led to Daly Cherry-Evans' 79th-minute try.
"For long periods there we showed a tonne of effort and gave ourselves a chance to get a result," coach Anthony Seibold said.
"Ultimately a couple of lapses let us down; three or four times where we missed the mark and they made us pay."
Manly are hopeful second-rower Haumole Olakau'atu only suffered a cork when he hurt his shoulder in the loss, while Cronulla's Addin Fonua-Blake has a groin issue.
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