St Patrick's Athletic will go into the second leg of their quarter-final with a one-goal advantage thanks to Craig Forrester's 21st-minute close-range finish at Ferney Park.
Pat's made the breakthrough when a Ger O'Brien throw-in was picked up by Keith Fahey and he rolled the ball across the box for Forrester to tap in.
The visitors went close to doubling their advantage 10 minutes later when Forrester rose to meet a Conan Byrne corner but he could not steer his header on target.
Pat's continued to press early in the second half with Killian Brennan having a free-kick saved by McCusker while Forrester had a second ruled out for offside.
The Mallards had a good chance to snatch a share of the spoils late on when Andy Crawford got in on goal in the 86th minute but he could not beat Rene Gilmartin.
Ballinamallard – McCusker, Stafford, Kee, Hutchinson, Lwcky, Hutchinson (Beacom 65), Lecky (Crawford 71), McCartney, Feeney, McManus (Currie 81), Campbell, Martin, Crilly.
St Pat's – Gilmartin, O'Brien, Bermingham, Oman, Browne, Byrne, Fagan, Fahey (Lynch 81), Brennan (Chambers 86), Forrester (Kavanagh 81), G Bolger.
Ref – M Courtney