© 2016 by the American College of Sports Medicine. Physical activity (PA) has numerous health benefits, particularly for those with diabetes. However, rates of long-term PA participation are often poor. Purpose This study examined the effect of an intensive lifestyle intervention (ILI) on objectively assessed PA for a 4-yr period among older adults with type 2 diabetes. Methods Data from 2400 participants (age = 59.3 ± 6.9 yr, body mass index = 36.1 ± 5.9 kg·m-2) with accelerometry data from the Look AHEAD trial were included in the analyses. Participants randomized to ILI were instructed to reduce caloric intake and progress to ≥175 min·wk-1 of moderate-to-vigorous-intensity PA (MVPA), whereas those randomized to Diabetes Support and Education (DSE) served as the control group. PA was measured at baseline, year 1, and year 4 using an RT3 accelerometer, and bout-related MVPA (PA ≥3 METs, accumulated in bouts of ≥10 min in duration) was calculated. Results Despite no differences at baseline (ILI = 93.4 ± 152.7 vs DSE = 88.4 ± 143.6 min·wk-1), bout-related MVPA was significantly greater in ILI compared with DSE at year 1 (151.0 ± 213.5 vs 87.5 ± 145.1 min·wk-1, P < 0.0001) and year 4 (102.9 ± 195.6 vs 73.9 ± 267.5 min·wk-1, P < 0.001), and more ILI participants achieved ≥175 min·wk-1 at year 1 (29.1% vs 16.3%, P < 0.001) and year 4 (18.3% vs 10.0%, P < 0.001). Forty-one percent of ILI participants who achieved ≥175 min·wk-1 at year 1 maintained this threshold of PA at year 4. However, the majority of ILI participants never achieved the ≥175 min·wk-1 threshold. Conclusions When measured objectively and compared with DSE, ILI engaged in significantly more bout-related MVPA for a 4-yr period. However, future intervention strategies should target the large percentage of individuals who fail to reach the MVPA goal as result of a lifestyle intervention.