We investigate the formation and evolution of giant molecular clouds (GMCs) by the collision of convergent warm neutral medium (WNM) streams in the interstellar medium, in the presence of magnetic fields and ambipolar diffusion (AD), focusing on the evolution of the star formation rate and efficiency (SFE), as well as of the mass-to-magnetic-flux ratio (M2FR) in the forming clouds. We find that (1) clouds formed by supercritical inflow streams proceed directly to collapse, while clouds formed by subcritical streams first contract and then re-expand, oscillating on the scale of tens of Myr; (2) our suite of simulations with the initial magnetic field strength of 2, 3 and 4 μ G show that only supercritical or marginal critical streams lead to reasonable star forming rates. This result is not altered by the inclusion of AD; (3) the GMC’s M2FR is a generally increasing function of time, whose growth rate depends on the details of how mass is added to the GMC from the WNM; (4) the M2FR is a highly fluctuating function of position in the clouds. This implies that a significant fraction of a cloud’s mass may remain magnetically supported, while SF occurs in the supercritical regions that are not supported; (5) in our simulations, the SFE approaches stationarity, because mass is added to the GMC at a similar rate to which it converts mass to stars. In such an approximately stationary regime, we find that the SFE provides a proxy of the supercritical mass fraction in the cloud; and (6) the low-M2FR regions exhibit buoyancy within the gravitationally contracting GMCs, so that the latter naturally segregate into a high-density, high-M2FR ‘core’ and a low-density, low-M2FR ‘envelope’, without the intervention of AD.