A botched play-the-ball by Luke Burt after just four minutes led to the first of Williams's tries and another fumble from Cheyse Blair 10 minutes later handed possession to the hosts for his second.
With former Eel Tony Williams and winger Jorge Taufua, a Wentworthville junior, tearing the visitors' right side to bits, the floodgates were blown open by the home side before halftime.
Taufua brushed aside a feeble tackle from Ken Sio to set up Brett Stewart for his sixth try of the year, before Williams added his third and fourth of the day, either side of a Daly Cherry-Evans four-pointer.
Stewart added his second six minutes into the second half, with Jamie Lyon's kick making it 40-0 after more feeble defending from Blair allowed the NSW fullback to stroll to the line.
The Sea Eagles then took their foot off the gas and the Eels took advantage.
Mullaney barged over after 56 minutes and winger Sio, one of the few positives for the Eels this year, scored twice in the right corner.
Burt added a fourth for the Eels on the siren before kicking his fourth conversion.
Jamie Lyon booted six goals for Manly.
-AAP