The Scorchers guard netted in the dying seconds as the Giants saw their 13-point half-time lead overhauled.
The visitors’ advantage was as much as 15 points but they could not hold on as a fourth BBL win of the season slipped away right at the death.
Mike Creppy’s team-high 23 points was a positive for the Giants – as was their dominant first-half performance.
But Margai winner was a dagger to the heart for Yorick Williams’ men who drop to a 3-14 record.
Callum Jones had 14 and Sam Toluwase 11 for the Giants.