Tuesday's vote puts the state's most populous county at odds with Republican state leaders who argue that pouring more money into Medicaid will mean sacrificing other funding priorities, such as education and transportation.
But conservative rural counties soon may follow suit.
"Salt Lake is the first to go on record as supporting the expansion, but I know other counties are considering what their positions will be," said Adam Trupp, legal counsel for the Utah Association of Counties.
Trupp says there's no consensus yet.
Salt Lake County has the most at stake because it accounts for more than 40 percent of Utah's 255,000 Medicaid enrollees, whose numbers could double under the expansion.
But counties everywhere face the same stark reality.
The federal government foots 100 percent of the expansion for the first three years. After that, the state and county have to pony up a gradually rising share, never to exceed 10 percent.
Officials estimate $5 million in state and local funds would buy Salt Lake County $50 million in mental health and substance abuse services that it's on the hook to provide, with or without reimbursement.
Utah law requires counties to treat anyone who seeks treatment within funding constraints.
Already they're struggling to care for clients with Medicaid, Trupp said, adding that the uninsured face indefinite waits for therapists, making counties liable to claims that they're not meeting their obligations under the law.
"If Medicaid doesn't expand and that requirement remains in state law, how are we going to fund services for those who don't have insurance?" Trupp asked.
Consumer advocates hope a push from counties will spark a sophisticated, numbers-driven debate on what has become one of the most controversial elements of the Affordable Care Act.
"As we see more states move forward with the Medicaid expansion, even Republican-led states like Nevada and Arizona, the momentum is building in Utah. County-level advocacy will be key in helping legislators understand the very significant benefits of the expansion," said Matt Slonaker, Medicaid policy and collaborations director at the Utah Health Policy Project.
In Utah, being poor isn't enough to qualify for Medicaid. You have to be poor and pregnant, or poor and a child, or poor and disabled.
Federal health reform would turn Medicaid into a purely needs-based program, benefiting "mainly the single adults who don't have children, people in the jail, in Pioneer Park," said county division of behavioral health services director Patrick Fleming.
Medicaid eligibility for the jail population alone would jump from 20 percent to 87 percent, Fleming added, noting in a slide presentation that there is significant evidence that inmates who have Medicaid upon release from incarceration are less likely to return to jail.
In that way, Salt Lake County District Attorney Sim Gill said, expanding Medicaid is consistent with efforts to provide more alternatives to incarceration.
"As this county has championed therapeutic justice, this allows us to expand the continuity of care for a group that is left out," Gill said, noting the expansion is supported by the Criminal Justice Advisory Council. "If we don't participate, the need won't dissipate, but the funds will."
Stoking the GOP-led County Council's fondness for the expansion was the prospect of seeing Utah's share of the federal tax base go toward expanding Medicaid in other states.
"This is a wise position for us to take," said Republican Councilman David Wilde.
The final say was left to new Democratic Councilman Sam Granato. "Let's step up to the plate and send a message loud and clear to the governor and Legislature that we need those federal dollars here."
Salt Lake County Council support for accepting future federal support for an expanded Medicaid program in Utah was based partly on figures showing how many more people will be eligible for coverage:
• About 100,000 Salt Lake County residents were on Medicaid in 2013.
• With changes in federal law, the number of Medicaid-eligible people is expected to grow to 144,560.
• Thirty to 40 percent of those extra people are likely to have behavioral health needs, generating considerable expenses that would be covered if the state joins the expanded Medicaid program.