This paper presents the measurement and analysis of a lateral-offset optical fiber Mach–Zehnder interferometer (MZI) as chloride ion concentration sensor using a near infrared light source (amplified spontaneous emission, wavelength = 1520-1620 nm). An 8-cm optical fiber MZI sensor was fabricated and fusion spliced using a lateral-offset process. We used this 8-cm lateral-offset op-tical fiber Mach–Zehnder interferometer (MZI) and measured chloride ions in samples of sodium chloride solutions with different weight concentrations ranging from 0.015% to 25% and then analyzed those interference spectra regarding normalized intensity, wavelength shift, and three ranges of the integral areas (1520-1580 nm, 1540-1600 nm, and 1520-1620 nm). The comparative spectral analysis results show that the lateral-offset optical fiber MZI sensor exhibited a linear decrease in the normalized intensity as well as wavelength shift when the concentration increased. The lateral-offset optical fiber MZI sensor displays a sine wave plot in the three ranges of integral areas when the concentration increased. Other than sensing parameters such as normalized intensity (adjusted R-squared = 0.98223) or wavelength shift (adjusted R-squared = 0.94209), the three ranges of integral areas (adjusted R-squared = 0.96425, 0.91621, and 0.9577, respectively) which possessed adjusted R-squared greater than 0.9, are also recommend as sensing parameters for this measurement and analysis of a lateral-offset optical fiber Mach–Zehnder interferometer (MZI) as chloride ion concentration sensor using a near infrared light source.