The Forthill side had home advantage and a further boost with the return of Scotland star Craig Wallace for his first appearance of the campaign.
However, the hard-hitting opening batsman was out for a duck, bowled by Brendon Ford, as Shire made the worst possible start.
A partnership between Matty Parker and Callum Garden put them back on track until Garden was out lbw to Craig Ramsay.
Parker went on to score a composed half-century against his former Arbroath team-mates while Chris Greaves chipped-in with 34 but the arrival of Calvin Burnett into the attack sparked the beginning of the end for the Broughty Ferry side.
Burnett claimed 4-25 while Ford returned to also pick up four wickets as Forfarshire faded to 176 all out.
In reply Ford continued a personal day to remember, reaching a rapid half-century as Arbroath raced to 60-0 in nine overs.
Ford departed soon after, miscuing a Brock Ditchmen delivery while the same bowler removed Ross McLean for 31.
Forfarshire’s bowlers kept chipping away but the visitors eased home by five wickets and with five overs to spare.
Grange remain joint top after a comprehensive win over city rivals Watsonians at Portgower Place.
Former Scotland seamer Gordon Goudie turned back the clock with a superb bowling display to claim 5-27 as the visitors were dismissed for 160.
Another ex-Saltire Ryan Flannigan then led the run chase with an unbeaten 69 to make sure Grange easily reached a revised target following a rain interruption.
Australian Michael Herdman maintained his rich vein of form with a brilliant century as Carlton dodged the showers to get the better of Aberdeenshire at Grange Loan.
SMRH hit a substantial 274-8 against Falkland at Scroggie Park and it proved enough for victory as the Fifers were behind on DLS when rain brought a premature end.
They persisted late into the evening at Goldenacre where Heriot’s won a last-over thriller despite a valiant unbeaten 51 from Glenrothes’ Scotland all-rounder Safyaan Sharif.
Rain was also a factor in the west where Ferguslie beat the weather and East Kilbride at Meikleriggs. Riyaad Henry’s 70 and 44 from Gregor Preston-Jones helped the Paisley side post 237-7.
Set a revised target of 242 from 46 overs, East Kilbride were firmly in contention until Henry’s 4-32 proved decisive in a 40-run success.
Kelburne remain winless after going down by seven wickets at Nunholm where Scott Beveridge claimed five wickets and Neil Alexander four to dismiss the visitors for 156.
Josh Forsyth hit 43no and Chris Bellwood 42no to seal a second win of the season for Dumfries.
Gavin Pitt and Sachin Chaudary claimed three wickets each as Prestwick restricted Poloc to 150 at Shawholm before rain interruptions saw the target revised. With the weather threatening again, the Ayrshire side eventually scraped to victory by three wickets.
No result was possible at Bothwell Castle Policies where champions Clydesdale were denied the chance to press home an early advantage against leaders Uddingston.