The benefits of growth‐stimulating microbes in crop production represent great opportunities for recent agricultural practices. Thus, the present investigation deals with examining whether arbuscular mycorrhizal (AM) fungi or Trichoderma viride application or their dual inoculation could improve the biochemical parameters and mineral and nutrient contents of onion plants (Allium cepa) under glasshouse conditions. The results evidenced that both AM fungi and T. viride are compatible with each other, and their combined use is effective, not only in improving the biochemical parameters, such as total soluble carbohydrates, protein contents, total free amino acids, acid, and alkaline phosphatases, but also in increasing mineral and nutrient contents (N, P, K+, Ca2+, Mg2+, and Zn) in onion plants, in which an increase of 67%, 49%, and 112% was observed in shoot onion P content with AM, T. viride, and with their dual inoculation, respectively, as compared with the controlled ones. Also, AM fungal colonization percentage augmented greatly with T. viride inoculation. Moreover, the protein profile of onion leaves revealed the appearance of newly protein bands with AM and T. viride applications. Therefore, their applications improved onion plant development, which could be used to replace the expensive chemical fertilizers, thus increasing onion quality.