Highly regioselective olefination of substituted N,N-dimethylbenzylamines was developed by tuning the acidity of reaction conditions based on analysis of their features. The ortho-functionalized N,N-dimethylbenzylamines were further transformed into 3-(2'-tolyl)propanoic acid and its derivatives under mild conditions. These two transformations could be combined into one pot, and 3-(2'-tolyl)propanoic acid and its derivatives were obtained in moderate to good yields. Mechanistic studies indicated that electrophilic attack on the phenyl ring by the Pd(II) ion assisted by the N,N-dimethylaminomethyl group was a key step during this catalytic transformation, which was controlled by the acidity of the reaction conditions.