In this work, 2D, steady, natural convection in a partially active/heated open ended square cavity subjugated to the magnetic field for incompressible, Newtonian fluid is studied and presented. A simplified double distribution function (DDF)-thermal lattice Boltzmann method (TLBM) based on single relaxation time (SRT) is utilized for solving field controlling equations. One of the vertical walls of open cavity is exposed to the heat source (i.e., heater) partially; while another vertical wall is open to ambient. In particular, the influence of various geometric as well as parametric conditions, such as heater size (LH = 0.25, 0.5, 0.75), angle of magnetic field (θM = 0° 45° and 90°), Hartmann number (0 ≤ Ha ≤ 100) and Rayleigh number (103 ≤ Ra ≤ 105) on local and global convection features have been investigated. The dependence of average Nusselt number with magneto-convective parameter (λ = Ha2/Ra) has also been illustrated. It is observed that cavity with the applied magnetic field at θM = 45° offers highest heat transfer restriction than other considered cases. Effect of heater size remains effective only at higher Ra (≥105). © 2017 Institution of Chemical Engineers