Short answer: For most people, yes. Marketplace and other individual and small-group plans must cover mental health and substance use treatment as an essential health benefit; large-employer plans usually do but are not required to.
MHPAEA by itself is a parity law, not a coverage mandate. It does not force a plan to offer behavioral health benefits, but it requires that any plan which does cover mental health and substance use disorders does so in a similar way to medical and surgical benefits. The Affordable Care Act supplies the coverage requirement for individual and small-group plans by listing mental health and substance use disorder services among the ten essential health benefits.
So if you buy a Marketplace plan or have small-group coverage, behavioral health treatment, inpatient mental health care, and substance use disorder treatment are included, pre-existing behavioral conditions are covered, and no annual or lifetime dollar limits may apply to them. If you have large-employer coverage, check the plan documents: the benefit is common but is not federally required as an essential health benefit.