Background. Depression, anxiety, and sleep problems are prevalent in high-stress occupations including military service. While effective therapies are available, scalable preventive mental health care interventions are needed. This study examined the impact of a combined mindfulness and yoga intervention on the mental health of soldiers in Basic Combat Training (BCT). Methods. U.S. Army soldiers (
N
=
1,896
) were randomized by platoon to an intervention or training-as-usual condition. Soldiers in the intervention condition completed Mindfulness-Based Attention Training (MBAT), engaged in daily 15 min mindfulness practice, and participated in 30 minutes of hatha yoga 6 days per week. Surveys were administered at baseline (T1, prior to training), week 4 of BCT (T2), week 6 (T3), and week 9 (T4). Results. A significant time-by-condition interaction predicting positive screens for depression found that screens decreased at a faster rate from T1 to T4 in the intervention condition (-12.6%) compared to training-as-usual (-7.2%) (
b
=
−
0.18
,
SE
=
0.07
,
p
=
0.028
). While positive anxiety screens decreased over time across conditions, the time-by-condition interaction found no significant differences in the rate of these decreases by condition (
b
=
0.09
,
SE
=
0.09
,
p
=
0.273
). A significant time-by-condition interaction predicting positive screens for sleep problems found that sleep problems decreased in the intervention condition (-1.4%) but increased in training-as-usual (2.0%) (
b
=
−
0.68
,
SE
=
0.16
,
p
=
0.027
). Conclusion. The mindfulness and yoga intervention was associated with a greater reduction in positive screens for depression and sleep problems among soldiers during high-stress training. Limitations include reliance on self-report and the inability to disaggregate the effects of mindfulness versus yoga. Mindfulness and yoga may enable personnel in high-stress occupations to sustain their mental health even in the context of significant psychological demands. This trial is registered with NCT05550610.