By Gabriel Tan at the Bishan Stadium
Despite Home entering the game as favourites, it was the visitors who took the lead when Jonathan Toto applied to finishing touches to a good team move after just four minutes.
Nonetheless, the Protectors hit back ten minutes later when Firdaus Idros slotted home from close range, before Mendy grabbed a second-half double to hand his side all three points.
Having played just three days earlier, Young Lions coach Robin Chitrakar opted to freshen up his squad by making three changes to the side that took to the field in the 2-0 win over Hougang United, one of which he was forced into after captain Hafiz Abu Sujad picked up his fourth yellow of the campaign against the Cheetahs.
However, his industrious presence in the middle of the park was hardly felt early on as Neil Vanu played a key role in the Young Lions' early opener. Picking up the ball just inside the opposition area, the 22-year-old played a neat pass down the right to release Benjamin Kristofferson Lee, who cut the ball back for Toto to fire home from 12 yards out.
Stunned at going behind so early on, the Protectors immediately looked to restore parity and came close in the 12th minute when Mendy played Firdaus through with an incisive through-ball. However, Al-Qaasimy Rahman did extremely well to dispossess the winger just as he was about to bear down on goal.
Nonetheless, there was to be no denying Home two minutes later as Mendy and Firdaus once again combined to good effect, with the former nodding a Qiu Li cross straight to the latter, who calmly slotted past Syazwan Buhari to level the scores.
Two minutes before the half-hour mark, it was Mendy's turn to make his way in behind the visitors' defence as he timed his run to perfection to latch onto another delivery from Qiu, although his decision not to fire a first-time shot away cost him as Al-Qaasimy rushed back well to block him out.
Qiu was starting to give Young Lions right-back Faritz Hameed all kinds of problems down the left with his pace and skill and he carved out another opportunity for the hosts in the 30th minute. After effortlessly barging over his direct opponent down the left, the Singapore international laid the ball back to the edge of the area where John Wilkinson proceeded to curl a delicate effort towards goal, only to see it bounce agonisingly back off the post.
By now, Chitrakar's young charges were really struggling to compete with their bigger-bodied opponents and four minutes after halftime, the Protectors finally went ahead after Franklin Anzite comfortably brushed off a challenge from Sheikh Abdul Hadi, before floating a cross straight onto head of Mendy, who made made no mistake with a powerful header at the far post.
Five minutes later, Young Lions had a decent chance to hit back when Vanu nodded a cross from Kristofferson Lee just wide, before Lionel Lewis did well to intercept a Toto flick-on header before substitute Fazli Ayob could pounce inside the six-yard box.
Sherif El-Masri, Young Lions' two-goal hero against Hougang earlier in the week, was having a fairly quiet game but he showed his quality in the 66th minute when he swung in a terrific cross towards the far post, only to see Fazli nod a free header into the ground and back up over Lewis' crossbar.
Watch: Highlights - Home United 3-1 Young Lions... More Videos
With their one-goal lead looking increasingly in danger of being cancelled out, the Protectors started to push more men forward and came close to scoring when Indra Sahdan Daud, who had came on at halftime, saw a well-struck effort cleared off the line by Faritz.
Buoyed by that close shave, the hosts continued to pile on the pressure and duly wrapped up the three points 12 minutes from time when captain Shi Jiayi floated a long ball into the opposition box. Despite being surrounded by a couple of defenders, Mendy was determined to reach the cross and comfortably outjumped Syazwan, before nodding home his second to seal the win for his side.
Home United: Lionel Lewis, Nor Azli Yusoff, Franklin Anzite, Sofiyan Abdul Hamid, Kenji Arai (Rosman Sulaiman 90'), Masrezwan Masturi, John Wilkinson (Indra Sahdan Daud 46'), Shi Jiayi, Firdaus Idros (Shotaro Ihata 36'), Qiu Li, Frederic Mendy.
Courts Young Lions: Syazwan Buhari, Faritz Hameed, Sheikh Abdul Hadi, Sirina Camara, Al-Qaasimy Rahman, Aqhari Abdullah, Shamil Sharif (Faris Ramli 63'), Neil Vanu (Fazli Ayob 55'), Benjamin Kristofferson Lee, Sherif El-Masri, Jonathan Toto (Syafiq Zainal 79').