Centre-half Cain took the gloves after Moneys stopper Callum McGeorge had been forced off through injury on 73 minutes.
The defender-turned-stand-in stopper was called into action on a couple of occasions before the final whistle, punching the danger clear from a corner and saving with his feet as stoppage-time approached.
But Cain was probably not as nervy as he might have been with Moneyfields already leading 2-0 when he was forced to take the gloves of the injured McGeorge.
In front of a sizeable midweek crowd of 260 supporters at Cams Alders, McGeorge made a couple of quickfire saves to prevent out-of-form Fareham - who have now collected just one point from a possible 27 on offer in their past nine league outings - going ahead inside 20 minutes.
Midfielder Jordan Pile was just unable to poke the visitors ahead then, just a minute later, Joseph Briggs fired into the bottom left corner to hand Moneys the lead after 41 minutes.
Talisman Steve Hutchings then took his season scoring tally to 26 goals in just 23 appearances to double Moneyfields' advantage by slotting past Henry Woodcock five minutes after the restart.
Fareham then offered a scare, so nearly pulling a goal back, just minutes before goalkeeper McGeorge had to be withdrawn due to injury.
Birmingham came on for him but Cain held his hand up to take the gloves - still with 17 minutes of normal time left on the clock.
But teenage Moneys striker Evan Harris would wrap up the victory, adding a third on 85 minutes, slotting past Woodcock. Moneyfields, who remain ninth in the table despite the victory, have now made it seven Wessex Premier matches without defeat as they beat Pete Stiles' side.