The use of the sole third finger middle phalanx for a maturational method has been proposed but not fully investigated. Herein, the diagnostic reliability of an improved five-stage third finger middle phalanx maturation (MPM) method in the identification of mandibular growth peak has been investigated. From the files of the Burlington Growth Study, 35 subjects (20 males, 15 females) with at least 7 annual lateral cephalograms taken from 9 to 16 years were included. Mandibular growth was defined as annual increments in condylion-gnathion (Co-Gn) distance. Subsequently, individual annual increments in Co-Gn were arranged according to annual age intervals, with the first and last intervals defined as 9-10 years and 14/15-16 years, respectively. A full diagnostic reliability analysis (including positive likelihood ratio) was performed to establish the diagnostic reliability of the MPM stage 2 (MPS2) in the identification of the imminent mandibular growth peak. The MPS2 had a satisfactory accuracy in the identification of imminent mandibular growth peak with an overall positive likelihood ratio of 10.3. However, reliability showed noteworthy variability being greater and lower for younger and older age intervals, respectively. Secular trend, limited sample size, and annual recording in conjunction with the use of a discrete staging system. At the 15 years recording, 28 of 35 cases were missing. The MPS2 and MPS3 may be considered associated with the onset and maximum mandibular growth peak, respectively, in most of the subjects, indicating their use in planning treatment timing.