We prove the principle of maximal transcendentality for a class of form factors, including the general two-loop minimal form factors, the two-loop three-point form factor of tr(F 2 ), and the two-loop four-point form factor of tr(F 3 ). Our proof is based on a recently developed bootstrap method using the representation of master integral expansions, together with some unitarity cuts that are universal in general gauge theories. The maximally transcendental parts of the two-loop four-gluon form factor of tr(F 3 ) are obtained for the first time in both planar N = 4 SYM and pure YM theories. This form factor can be understood as the Higgs-plus-four-gluon amplitudes involving a dimension-seven operator in the Higgs effective theory. In this case, we find that the maximally transcendental part of the N = 4 SYM result is different from that of pure YM, and the discrepancy is due to the gluino-loop contributions in N = 4 SYM. In contrast, the scalar-loop contributions have no maximally transcendental parts. Thus, the maximal transcendentality principle still holds for the form factor results in N = 4 SYM and QCD, after a proper identification of the fundamental quarks and adjoint gluinos as n f → 4N c . This seems to be the first example of the maximally transcendental principle that involves fermion-loop contributions. As another intriguing observation, we find that the four-point form factor of the half-BPS tr(φ 3 ) operator is precisely a building block in the form factor of tr(F 3 ). C Two-loop symbol letters and collinear limit 54 C.1 Symbol letters 54 C.2 Collinear limit of the letters 56 D Constraints from higher order of ǫ-expansion 57 E Building-blocks for 2-loop four-point form factor of tr(F 3 ) 59 F Remainders and numerical results for the four-point form factor 62 G One-loop four-gluon amplitude: a counter-example of MTP 63 23 12 23 23 23 12 , 1 − 1 w = 2 312 23 12 23 2 3 .(2.16)These variables are enough to provide the symbol letters for the remainder functions of threepoint form factors of tr(F 2 ) [5,47]. In this case, the collinear limit is relatively trivial, for example, in the limit of p 1 p 3 , one has s 13 → 0, q 2 → s 12 + s 23 and u → u, v → 1 − u, w → 0 .