Jamie Heywood’s players went down by nine wickets at Lune Road on a day when the batters failed to fire and the visitors breezed to their target.
Rizwan Patel struck early, removing Heywood (5) and Laurie Atkinson (0).
Ian Walling also got in on the act, picking up the wicket of Eddie Steinson (5).
Irfan Khan and Jack Lord attempted to rally, the former top-scoring with 25 before he was out to Danny Gilbert.
Joel Derham saw off Lord (19) before Gilbert ran through the Lancaster middle and lower order.
The Garstang captain got the better of Steven Fisher (3), Ben Simm (2), Kieran Moffat (0) and David Steinson (0).
Liam Moffat was the final man to fall, dismissed by Michael Walling for 21, leaving last man Lee Sparks one not out.
Gilbert topped the Garstang bowling figures with 5-17, while Patel picked up 2-31, Michael Walling 1-5, Ian Walling 1-13 and Derham 1-15.
In reply, the Garstang opening pair of Michael Wellings and Michael Walling set up their run chase.
Wellings had struck five fours and a six in reaching 35 before he was dismissed by Ben Simm.
Michael Walling ended 32 not out and Punit Bisht 19 not out as Garstang reached 93-1 midway through the 22nd over.
Simm finished with 1-30 as second-placed Garstang picked up a maximum 15-point haul to keep leaders Blackpool within range.
Defeat, Lancaster’s fifth in seven games, sees them in ninth ahead of Saturday’s trip to St Annes (12.30pm), who sit fourth after an unbeaten start.
Sunday then brought defeat in Lancaster’s latest Readers T20 group stage match as they lost by two wickets at Penrith.
After winning the toss, Lancaster chose to bat first and concluded their 20 overs on 99-8.
Eddie Steinson top-scored with 24 as Greg Cameron took 2-10 and James Ellis 2-15.
Penrith’s reply saw them reach 100-8 with one ball remaining despite Heywood taking 3-20.
Asher Hart led their run chase with 27, while Owen Jones added 23 as they claimed a first group stage victory.