Embodiments of the present disclosure are directed to sealant compositions including a base composition with at least one sulfur-containing polymer, a curing agent composition, and an electrically conductive filler including carbon nanotubes and stainless steel fibers. The electrically conductive filler can be in either or both of the base composition and the curing agent composition. The sealant compositions are substantially Ni-free and exhibit unexpectedly superior EMI/RFI shielding effectiveness.