UnitedHealth is among the largest players in the Medicare Advantage market, where private insurers offer an alternative to the original Medicare - the federal government's health insurance plan for people aged 65 and older or those with certain disabilities.
Medicare and Medicaid memberships make up a third of the company's health insurance business.
It added 655,000 Medicare Advantage members and 570,000 Medicaid members in the quarter.
Meanwhile, a slow recovery in non-urgent procedures helped lower medical costs at its insurance unit, driving the company's first-quarter profit above expectations.
The company's medical cost ratio - the percentage of payout on claims compared with premiums - came in at 82.2%.