Abstract We present results from deep Spitzer/Infrared Array Camera (IRAC) observations of 28 metal-poor, strongly star-forming galaxies selected from the DEEP2 Galaxy Survey. By modelling infrared and optical photometry, we derive stellar masses and other stellar properties. We determine that these metal-poor galaxies have low stellar masses, M⋆ ≈108.1–109.5M⊙. Combined with the Balmer-derived star formation rates (SFRs), these galaxies have average inverse SFR/M⋆ of ≈100 Myr. The evolution of stellar mass–gas metallicity relation to z ≈ 0.8 is measured by combining the modelled masses with previously obtained spectroscopic measurements of metallicity from [O iii] λ4363 detections. Here, we include measurements for 79 galaxies from the Metal Abundances across Cosmic Time Survey. Our mass–metallicity relation is lower at a given stellar mass than at z = 0.1 by 0.27 dex. This demonstrates a strong evolution in the mass–metallicity relation, $(1+z)^{-1.45^{+0.61}_{-0.76}}$. We find that the shape of the z ≈ 0.8 mass-metallicity relation, a steep rise in metallicity at low stellar masses, transitioning to a plateau at higher masses, is consistent with z ∼ 0.1 studies. We also compare the evolution in metallicity between z ≈ 0.8 and z ∼ 0.1 against recent strong-line diagnostic studies at intermediate redshifts and find good agreement. Specifically, we find that lower mass galaxies (4 × 108M⊙) built up their metal content 1.6 times more rapidly than higher mass galaxies (1010M⊙). Finally, we examine whether the mass–metallicity relation has a secondary dependence on SFR, and statistically concluded that there is no strong secondary dependence for z ≈ 0.8 low-mass galaxies.