Thomas hit 9 of 18 from the field, including four 3-pointers, while Rhoomes made 9 of 11, grabbed 10 rebounds and blocked six shots.
Christian Sengfelder contributed 16 points, Joseph Chartouny had 11 and Antwoine Anderson 10 for Fordham (4-1), which shot 56 percent from field and was 10 of 19 from 3-point range.
Sengfelder's 3-pointer late in the first half sparked a 13-3 run stretching across both sides of the break to make it 46-33 with 18:35 left. The margin remained in double figures the rest of the way as the Rams continued to extend their advantage.
Manhattan (1-3) got 24 points from Rich Williams and 13 from Tyler Wilson, who made 9 of 14 free throws.