Manganese clusters are formed by laser vaporization in an inert gas condensation source cooled to -160 degrees C. A pure manganese target is used as well as a manganese target containing 2 mol % carbon. The clusters are reacted with hydrogen both in the region of cluster growth and in a flow-tube reactor (FTR) downstream of the cluster source. The reactions, both with hydrogen atoms in the cluster growth region and dissociative chemisorption of H-2 in the FTR, result in hydrogen atoms bound to the clusters, except that for Mn-15 and smaller clusters the H atoms are unstable against H-2 desorption. Above Mn-15 stable hydrides are formed, but the rate of reaction in the FTR varies considerably with cluster size. This abrupt change in the ability to bind hydrogen may reflect a significant change in the character of the bonding within the cluster, perhaps from van der Waals to metallic. MnnC clusters readily react with hydrogen for n>6 to form MnnCH2. Further reactivity generally follows the pattern of the bare clusters, suggesting that the two hydrogen atoms on MnnCH2 are bound to the carbon. Under certain conditions a large intensity of MnnCH3 species (n greater than or equal to 1) is seen. From these results and earlier reports of reactions of Mn with various hydrocarbons, an energy of 1.21 +/- 0.09 eV is determined for the Mn-CH3 bond.