A fighting display from Miles Rutherford's Chichester City with an exciting young side made it a tough game for Bognor. But the Rocks were just too strong in the end.
City's Matt Axell clashed with Mongoy in a 50/50 challenge and he was holding his shoulder after the incident whilst received treatment.
Ashton Leigh burst into the box before being tripped in the area and the linesman flagged straight away for a penalty. Leigh stepped up to shoot low towards the bottom corner but it was pushed away by the keeper on seven minutes, keeping the game goalless.
Bognor were doing most of the attacking and went close a number of times before Gicu Iordache did well to dig out a cross down the other end, finding Callum Overton who glanced a header wide from inside the box on 36 minutes. But the linesman had his flag up for offside so a goal would not have counted.
Just before half time Calvin Davies did well to work his way into the area. He went to ground but claims for a penalty were ignored. But Jason Parish was clipped in the area straight after this and the referee pointed straight to the spot. Up stepped Mongoy who took his penalty well on 43 minutes, smashing it high into the net to give Bognor a deserved lead. HT 1-0.
Lots of substitutions took place at half time for Bognor. Harvey Whyte, Tyrone Madhoni, Phil Brown, Kayne Diedrick-Roberts and Ben Mendoza all came on. The last subs came for Bognor after the hour mark as Robson, Davies and Dandy made way for Jake Flannigan, Chad Field and Joe Hancott on 63 minutes.
Bognor got their second goal when Mendoza's ball forward to Diedrick-Roberts saw him stay composed, before beating Young with a fine strike, looping it into the net on 75 minutes.
Joe Cook went down off the ball and had to be taken off for Dandy, who had come off earlier in the game, on 82 minutes. Overton did well to beat his man before smashing it with his left foot off the upright and back out on 87 minutes in a rare chance for the visitors.
Another goal for Bognor came right at the death. A pinpoint cross by Flannigan found Mongoy for a simple tap in from the heart of the box on 90 minutes to end a comfortable evening at Nyewood Lane.
Rocks: Tangara, Dandy (Flannigan 45), Davies (Hancott 45), Robb (Mendoza 45), Cook (Dandy 82), Robson (c) (Field 64), McCallum (Whyte 45), Bell (Brown 45), Parish (Madhoni 45), Leigh (Diedrick-Roberts 45), Mongoy.
Chi City: Young, Dunn, Hutchings, Axell, Hyde, Pashley, Haitham, Horncastle, Overton, Prichard, Iordache. Subs: Biggs, Vuvac, Davidson, Oakwell, Winberger, Masters.