This paper studies the Mittag-Leffler stability and synchronization of fractional neural networks with multi-delays (FNNMs). First, by using the weak additivity Eα(δt2α)Eα(δt1α)≤Eα(δ(t1+t2)α)\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$E_\alpha (\delta t^\alpha _2)E_\alpha (\delta t_1^\alpha )\le E_\alpha (\delta (t_1+ t_2)^\alpha )$$\end{document} of the Mittag-Leffler function, the classical Halanay inequality is extended to a more comprehensive version cD0αr(t)≤κ(t)r(t)+χ(t)r(t-τ(t))+∫t-ρ(t)tG(t,s)r(s)ds+c(t)\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$$^cD_0^\alpha r(t)\le \kappa (t)r(t)+\chi (t)r{(t-\tau (t))}+\int _{t-\rho (t)}^t{G(t,s)r(s)} \,\textrm{d} s+c(t)$$\end{document}, which is critical to the stability assessment research for fractional neural networks (FNNs). Based on the new inequality, a nonlinear matrix inequality (NMI) criterion for the Mittag-Leffler stability of FNNMs is derived. The nonlinear matrix criterion can be solved step by step with the linear matrix inequality (LMI) toolbox, thus avoiding the limitations of searching LMI criteria. In addition, a Mittag-Leffler synchronization criterion in terms of NMI is presented for master-salve FNNMs under an adaptive feedback controller. Finally, there are three numerical examples as a demonstration of the validity of the proposed method. [ABSTRACT FROM AUTHOR]