Methylmalonic acid (MMA) plays a vital role in metabolism and energy production. It has been studied and reported as a sensitive early indicator for mild or serious Vitamin B12 deficiency. The normal range of MMA in blood among healthy people is from 0 up to 0.40 µM. Majority of MMA research was originally focused on Vitamin B12 deficiency with this small detection range. Recently, MMA has been reported to promote tumor progression due to age-induced accumulation. It was found that MMA concentration can reach as high as 80 µM in elderly people. Therefore, MMA can be a promising biomarker for cancer diagnostics, as well as a therapeutic target for cancer treatment. Clinical determination of MMA concentration in blood is normally executed by gas chromatography mass spectroscopy (GCMS) or liquid chromatography mass spectroscopy (LCMS). However, these methods require extensive sample pre-treatment, large sample volume and skilled lab personal. They are also expensive and time-consuming. Hence, we proposed an attractive and effective strategy to detect MMA with a broad linear range by a low-cost molecularly imprinted polyaniline paper sensor. The polyaniline paper strip was fabricated by a one-step solution process using MMA as the template by molecular imprinting technology. The concentration of MMA was determined by the resistance change of the paper sensor by a portable multimeter wirelessly. A calibration curve as a function of MMA concentration in aqueous solution was acquired with a correlation coefficient of 0.968. We also demonstrated detection of the added MMA in plasma with a wide concentration range of 0 to 100 µM with a limit of detection (LOD) of 0.197 µM. This low-cost disposable paper sensor has great potential in point-of-care MMA detection for cancer prognostics and diagnostics, especially in underserved communities.