Four cultivars of basil (Ocimum basilicum L. ‘Cinnamon’, ‘Siam Queen’, ‘Sweet Dani’, and ‘Red Rubin’) were inoculated or not with the arbuscular mycorrhizal fungus (AMF), Rhizophagus (formerly Glomus) intraradices (Schenck & Smith) Walker & Schüßler and grown with a fertilizer containing either 64 mg·L−1 phosphorus (P) (low P) or 128 mg·L−1 P (high P) to assess whether 1) P availability and inoculation with AMF influences the phenolic composition of basil; and 2) treatment effects on phenolic composition are related to plant nutrient status. Growth, root colonization by AMF, anthocyanins, total phenolics, specific polyphenolics, and mineral nutrients were measured after 16 weeks of growth. Non-inoculated plants were not colonized by AMF. AMF colonization of inoculated plants was not influenced by P rate. Increased P rate and AMF inoculation increased biomass. Increased P rate enhanced (increased concentration and content) P and calcium (Ca) uptake and AMF inoculation enhanced nitrogen (N), potassium (K), sulfur (S), boron (B), iron (Fe), and zinc (Zn) uptake. Increased or decreased uptake (content) of other nutrients between P rates and AMF treatments were related to differences in biomass (e.g., similar or lower concentration). Treatment effects on phenolic accumulation were related to the effects of P rate and AMF on 1) plant growth; 2) nutrient uptake; and 3) other factors not directly related to measured differences in nutrient uptake or plant growth. Differences between treatments in rosmarinic acid, the predominant polyphenolic produced by all cultivars, were related to the effects of P rate and AMF on plant growth. Both increased P rate and AMF inoculation enhanced production (increased concentration and content) of chicoric acid and caffeic acid derivative. Increased P rate and inoculation with AMF differentially enhanced production of several other minor polyphenolics resulting in plants with different polyphenolic profiles. Results indicate that AMF inoculation may be an additional strategy for optimizing basil quality in terms of polyphenolic production and composition beyond benefits obtained from just altering plant nutrient status or selecting specific cultivars.