Catalyzes the endo hydrolysis of beta-1,4-linked mannan and galactomannan, but displays little activity towards other polysaccharides located in the plant cell wall (PubMed:11382747). Preferentially hydrolyzes the larger oligosaccharides and has greater activity against non-substituted polysaccharides (PubMed:7848261) (PubMed:8973192). It displays tight specificity for mannose at both the -2 and the -1 subsites (PubMed:19441796). Appears to act in synergy with alpha-galactosidase (AgaA) to elicit hydrolysis of galactomannan (PubMed:11064195). Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. kcat is 14.7 sec(-1) for mannanase activity with 2,4-dinitrophenyl-beta-mannobioside (2,4-DNPM) as substrate (PubMed:8973192). kcat is 20 sec(-1) for mannanase activity with 2,4-dinitrophenyl-beta-mannobioside (2,4-DNPM) as substrate (PubMed:11382747). kcat is 1759 sec(-1) for mannanase activity with carob galactomannan as substrate (PubMed:8973192). kcat is 2904 sec(-1) for mannanase activity with carob galactomannan as substrate (PubMed:11382747). kcat is 3861 sec(-1) for mannanase activity with mannotetraose as substrate (PubMed:8973192). kcat is 280000 min(-1) for mannanase activity with galactomannan as substrate (PubMed:19441796). kcat is 400000 min(-1) for mannanase activity with glucomannan as substrate (PubMed:19441796). Optimum pH is about 7.0. Homodimer. Belongs to the glycosyl hydrolase 26 family.