He capitalised on a Sandesh Jhingan howler to strike home the lead -- and the eventual winner -- in the 61st minute.
Ferrando bolstered his defence in the final minutes, bringing in Brendan Hamill and Liston Colaco to stave off the wave of attacks from the Goans.
Goalkeeper Vishal Kaith also should be credited for saving a sure-shot goal when he thwarted a Jay Gupta header in the 90+2nd minute.
This was Mohun Bagan's first Durand Cup final after 2019 when they lost to Gokulam Kerala.
In 2004 also, Mohun Bagan had lost to East Bengal 1-2. They are yet to win the silverware since 2000 when they defeated Mahindra United via golden goal.
Both Mohun Bagan and East Bengal have won the Durand Cup 16 times each.
The Manolo Marquez-coached Gaurs looked far superior both in the midfield and defence, dishing out a compact play.
The FC Goa coach had a defence-heavy tactic against the home side, pressing five in the backline with Spaniard Odei Zabala, Seriton Fernandes seamlessly helming together the defence with Jhingan, Jay Gupta and defensive midfielder Car McHugh.
Juan Ferrando, on the other hand, went in for a 4-1-4-1 formation with Jason Cummings upfront.
Ashish Rai returned to replace Manvir Singh, while Sadiku was benched, making way for a fit-again Dimitri Petratos.
FC Goa also piled on the pressure early on and looked to go one up with Victor Rodriguez's measured freekick. However, Mohun Bagan's defence managed to clear the ball.
Mohun Bagan's first chance came in the 17th minute when Petratos' well-directed shot from a Boumous square pass was thwarted by Goa goalkeeper Dheeraj Singh.
The impressive Boumous turned out to be the villain as his misplaced pass from the midfield gave the Gaurs the early opening.
Sadaoui seized the opportunity with utmost calm as he surged ahead, with Mohun Bagan's defence putting up a half-hearted effort, before slotting past Kaith.
Goa could have easily gone 2-0 up in the 38th minute when Subhasish Bose appeared to have handed the ball inside the box from a Sadaoui strike but the penalty appeal was declined.
Moments later luck smiled on the Mariners as the referee gave a penalty in favour of the home side in what appeared to be a dubious decision.
Ashique Kuruniyan was tackled by Jay Gupta just outside the box before the Mohun Bagan midfielder tumbled inside the box and the referee awarded the penalty.
Cummings, with his left-footed strike, made no mistake to slot it past Dheeraj to bring in the equaliser.