Anterior mitral valve leaflet (AMVL) elongation is a recognised feature of hypertrophic cardiomyopathy (HCM). However, whether AMVL elongation precedes left ventricular hypertrophy in cats is currently unknown. The aim of this study was to explore the risk of developing an HCM phenotype in cats with an elongated AMVL. This was a retrospective longitudinal study. Cats at the baseline were grouped based on whether or not they developed an HCM phenotype at follow-up. AMVL length and left atrial and left ventricular dimensions were measured from two-dimensional images. The median follow-up period of the study population was 5.4 years (25th and 75th quartile, 2.7–6.7 years). During this time, 17 cats (30.9%) developed an HCM phenotype. At the baseline, cats that subsequently developed an HCM phenotype had greater AMVL length (9.4 mm 25th and 75th quartile, 9.0–10.6 mm vs. 8.5 mm 25th and 75th quartile, 7.6–9.1 mm, P < 0.0001) and maximal left ventricular wall thickness (4.5 mm 25th and 75th quartile, 4.1–4.7 mm vs. 4.0 mm 25th and 75th quartile, 3.7–4.6 mm, P = 0.007) than those that did not. Multiple logistic regression analysis confirmed that both baseline variables were independent predictors for development of an HCM phenotype. The AMVL length was greater in cats that subsequently developed left ventricular hypertrophy. Further studies investigating the clinical application of AMVL in the natural history of feline HCM are warranted.