In the present paper, for a pair (G, N ) of a group G and its normal subgroup N , we consider the mixed commutator length clG,N on the mixed commutator subgroup [G, N ]. We focus on the setting of wreath products:(G, N ) = (Z ≀ Γ, Γ Z). Then we determine mixed commutator lengths in terms of the general rank in the sense of Malcev. In particular, for an abelian group Γ that is not locally cyclic, we show that the ordinary commutator length clG does not coincide with clG,N on [G, N ]. On the other hand, we prove that if Γ is locally cyclic, then for every pair (G, N ) such that 1 → N → G → Γ → 1 is exact, clG and clG,N coincide on [G, N ]. We also study the case of permutational wreath products when the group Γ belongs to a certain class related to surface groups.