Assembly structure of Mn1-xNixFe2O4 (0<x<1) nanoparticles has been adjusted by introducing polyethylene glycol (PEG) into the synthesis procedure. Colloidal nanocrystal assembly (CNA) of Mn0.5Ni0.5Fe2O4 (M1N1-PEG) with inverse spinel structure can be formed with the assistance of PEG, whereas other CNAs of Mn1-xNixFe2O4 (0<x<1) with normal spinel structures are formed no matter PEG is added or not. The structural evolution of M1-xNixFe2O4 CNAs can be confirmed by the Raman spectroscopy and magnetic measurements as well as the theoretical simulation. Inverse spinel M1N1-PEG and NiFe2O4 CNAs only display an electrocatalytic peak toward the oxidation of L-histidine with high sensitivity in aqueous sodium sulfate electrolyte. While all the Mn1-xNixFe2O4 CNAs (0≤x<1) with normal spinel structure demonstrated the intercalation/deintercalation of sodium ions except for the inverse spinel structured M1N1-PEG.