The effects of ionic interaction on the uniaxial extensional flow of polymer melts were investigated using poly(ethylene-co-methacrylic acid) and its metal (Zn, Na, Mg) salts. The presence of ionic bonding enhanced the elongational viscosity in the linear region and the degree of non-linearity. Both the linear and non-linear elongational viscosities were increased more by zinc ions than by sodium or magnesium ions. The order of elongational viscosities was in agreement with the order of activation energies obtained from dynamic shear measurements. It was concluded that the increase in both linear elongational viscosity and non-linearity is determined by the degree of ionic interaction. The different flow properties between zinc and magnesium ionomers indicated that the ionic cluster formation weakens the ionic interaction. A Bernstein-Kearsley-Zapas constitutive analysis revealed that the effect of ionic interaction under elongational deformation in the linear region is similar to that under dynamic shear deformation.