A simple, rapid, and sensitive UHPLC-UV-MS method was developed for the quantitative determination of betaine (1), choline (2), acetylcholine (3), and 20-hydroxyecdysone (4) from various species of Atriplex. The baseline separation of the four analytes was achieved on a reversed phase C18 column within nine minutes. The mobile phase was composed of 50 mM ammonium formate in 2% methanol-water containing 5 mM sodium dodecyl sulfate (pH = 8.2) and methanol with 0.01% ammonium hydroxide. The analytical method was validated for recovery, precision, limits of detection (LOD), and limits of quantification (LOQ). The developed method was applied for the characterization and quantitation of analytes from plant parts of different Atriplex species, including A. canescens, A. fruticulosa, A. fasciculata, A. semibaccata, and A. lentiformis. Compounds 1–4 were found in a range of 0.53–1.61%, detection under limit of quantification (DUL)-0.74, DUL-0.0038, and 0-0.10% (w/w, mg in 100 mg plant material), respectively, in test samples. In leaf and fruit of A. canescens, a high content of 1, 2, and 4 were identified. The content of 1, 2, and 4 in A. canescens explains the potential implications of this native US plant for human health and nutrition. The result of this study provides a new method to analyze these phytoconstituents simultaneously in a mixture.