Mongia, 31, who has played 57 one-day internationals for India, made 59 and took 3/82 for East Grinstead in their match against Hastings but he failed to reverse their unhappy form after they fell 18 runs short of the 236 for seven target.
James Beeny struck 85 to keep Hastings in contention in second place, eight points behind leaders Horsham who managed a
draw against Three Bridges in a rain affected match at home.
Grinstead who leads the league table from the bottom, have their task cut out in the remaining ties. They need to come up with big effort to avoid relegation.