Such was their dominance, the Storm should have had the match wrapped up by halftime but only led 12-6 thanks to some uncharacteristically wayward kicking by their skipper Cameron Smith and Lyon's try in the 37th minute.
But Cronk's second, coming four minutes into the second half, saw Manly heads drop again.
The torrid semi-final win over North Queensland last weekend looked to have taken its toll on the visitors, who let themselves down with a barrage of handling errors and some lazy marker defence.
After two controversial decisions by the video referee favoured Manly against the Cowboys, they were on the receiving end at the nine minute mark on Friday.
Melbourne had already opened the scoring five minutes into the match through Cronk and they mounted another charge with centre Will Chambers tackled just short of the line.
Slater picked up the ball from dummy half and dived across the line with video referees Russell Smith and Sean Hampstead ruling a "benefit of the doubt" try despite the star fullback appearing to knock it on.
After being bundled out at the same point last year, Melbourne made no mistake with this opportunity, piling on the pressure as the Sea Eagles continued to implode.
Hard-working Storm forwards Jesse Bromwich and Kevin Proctor were both rewarded with tries.
Skipper Smith, who was rested in the final five minutes, said he couldn't have asked for more.
"It's very pleasing to get 40 points so it's a great win at home," Smith said from the sideline.
- AAP