Age at first calving (AFC) and milk yield (MY; accumulated 305-d milk yield) are important traits for the dairy cattle industry. To genetically improve these traits under tropical conditions in Thailand, both imported Holstein (IH) and Thai Holstein (TH; purebred and crossbred Holstein-Other Breeds) sires have been used for mating with Holstein purebred and Holstein-Other Breeds crossbred cows. The objective of this study was to compare genetic changes in AFC and MY from 1991 to 2014 in first-lactation daughters of IH and TH sires. The data set included AFC and first-lactation MY from 5,390 daughters of 507 IH and 232 TH sires that calved from 1991 to 2013 in 481 dairy farms. The bivariate animal model considered herd-year-season and Holstein fraction as fixed effects, and animal and residual as random effects. Variance components were estimated using an average information restricted maximum likelihood procedure. Variance component estimates were used to compute heritabilities and correlations. Mean estimated breeding value (EBV) for daughters of IH and TH sires were computed for each calving year. Regressions of mean EBV for AFC and MY on calving years were computed to assess genetic trends. Heritability estimates were 0.19 ± 0.04 for AFC and 0.13 ± 0.04 for MY. Near zero genetic (0.09 ± 0.18) and phenotypic (0.07 ± 0.02) correlations were estimated between AFC and MY. Overall genetic trends were negative for AFC (−0.013 ± 0.003 mo/yr; P < 0.001) and positive for MY (1.16 ± 0.30 kg/yr; P < 0.001). However, non-significant negative AFC genetic trends existed for daughters of IH sires (−0.006 ± 0.003 mo/yr; P = 0.06) and TH sires (−0.020 ± 0.004 mo/yr; P < 0.001). Conversely, significant positive MY genetic trends occurred for progeny of IH sires (1.14 ± 0.43 kg/yr; P = 0.008) and TH sires (4.40 ± 0.46 kg/yr; P < 0.001). These results indicated that there was genetic improvement in the appropriate direction (negative for AFC and positive for MY) in this Thai dairy population, and that TH sires brought larger changes than IH sires, primarily for MY.