Multiple carboxylase deficiency (MCD) is an autosomal recessive metabolic disorder caused by defective activity of biotinidase or holocarboxylase synthetase (HLCS) in the biotin cycle. Clinical symptoms include skin lesions and severe metabolic acidosis. Here, we reported four unrelated Thai patients with MCD, diagnosed by urine organic acid analysis. Unlike Caucasians, which biotinidase deficiency has been found to be more common, all of our four Thai patients were affected by HLCS deficiency. Instead of the generally recommended high dose of biotin, our patients were given biotin at 1.2 mg/day. This low-dose biotin significantly improved their clinical symptoms and stabilized the metabolic state on long-term follow-up. Mutation analysis by polymerase chain reaction-sequencing of the entire coding region of the HLCS gene revealed the c.1522C>T (p.R508W) mutation in six of the eight mutant alleles. This suggests it as the most common mutation in the Thai population, which paves the way for a rapid and unsophisticated diagnostic method for the ethnic Thai. Haplotype analysis revealed that the c.1522C>T was on three different haplotypes suggesting that it was recurrent, not caused by a founder effect. In addition, a novel mutation, c.1513G>C (p.G505R), was identified, expanding the mutational spectrum of this gene.