In-situ composites are a class of composite materials in which the reinforcing phases (such as Al2O3, TiB2,TiC, etc.) are generated within the matrix material by some chemical reaction during the composite processing. This research paper concerns comparison between microstructure, mechanical and tribological properties of the resulting cast in-situ Al(Mg,Mn)-Al2O3(MnO2) and Al(Mg,Ti)-Al2O3(TiO2) composites have been investigated. It is generally observed that intermetallic phase Mn(Al1-x Fex)6 in the cast in-situ Al(Mg,Mn)-Al2O3(MnO2) composite is relatively finer in size and is sometimes blocky type compared to Ti(Al1-x Fex)3 formed in cast in-situ Al(Mg,Ti)-Al2O3(TiO2) composite. This has been attributed to difference in heterogeneous nucleation behavior of the alumina substrates for the formation of intermetallic phases. Superior mechanical properties, as indicated by ultimate tensile stress, yield stress and percentage elongation, are obtained in the cast in-situ Al(Mg,Mn)-Al2O3(MnO2) composite compared to those obtained in cast in-situ Al(Mg,Ti)-Al2O3(TiO2) composite.It is observed that the wear rate in cast in-situ Al(Mg,Mn)-Al2O3(MnO2) composite is considerably lower compared to that of the cast in-situ Al(Mg,Ti)-Al2O3(TiO2) composite, particularly at higher normal load of 39.2 N, in spite of a relatively higher porosity content and slightly lower hardness in cast in-situ Al(Mg,Mn)-Al2O3(MnO2) composite.