Broadly neutralising monoclonal antibodies (bNAbs) targeting conserved epitopes on Human Immunodeficiency Virus (HIV) envelope (Env) are effective antiviral therapeutics. We made an ultra-potent humanized bNAb (MEL-1872) with distinctive ultra-long CDRH3 regions of 57aa from bovine that was repeatedly vaccinated with a series of different stabilised Env-trimers1. MEL-1872 binds the highly conserved CD4 binding site on HIV Env (IC50 0.009 mg/mL) without cross-reactivity to human cellular antigens. We explored mRNA delivery for expression of this bNAb antiviral preventing infectious spread and clearing HIV-infected cells. Sequence optimized immunoglobulin heavy (IgH) and light (IgL) chain plasmid templates were used for T7-RNA-pol in vitro transcription of pseudouridine-modified mRNAs incorporating a 5′ Cleancap, optimized 5′ and 3′ UTR, and a poly(A) tail. Stoichiometrically matched mixtures of purified IgH and IgL mRNAs achieved efficient expression of potent HIV-neutralising MEL-1872 after transfection into human and mouse cell-lines. We further engineered the IgG1 heavy chain2 to incorporate: a) the LS mutation binding neonatal Fc-receptor (FcRN) imparting systemic longevity from uptake recirculation; b) GASDALI mutation for FcgRIIA and FcgRIIIA binding for ADCC and NK-cell targeting; c) GASDALI/LS dual mutants, and d) substituting IgG1H for the IgAH1&2 and J-chain for dimeric dIgA engagement of poly-IgA-receptor imparting mucosal secretion. The suitability of mRNA delivery of MEL-1872 for viral neutralisation and cell-mediated antiviral activity was assessed in vitro using cell models including HIV productive or latent-infected cells. The mRNA mixtures were formulated into lipid nanoparticles (LNPs) tuned for either vaccination or immune-tolerant expression and delivered to ex vivo HIV-infected human T-cells or mice. Engineered MEL-1872 bNAb expression and potency for HIV neutralisation was confirmed from cell supernatant or mouse serum and mucosal washings. The anti-drug antibody responses against the distinctive bovine CDRH3 was assessed for different formulations to assess suitability of these mRNA-LNP formulations for clinical applications in HIV treatment and prevention.