Measure A was being backed by about 54.5 percent of voters, with just under 1/3 of precincts reporting.
A simple majority was needed for incorporation to be approved.
The 44-square-mile area includes Belltown, Glen Avon, Indian Hills, Jurupa, Mira Loma, Pedley and Sunnyslope.
The measure was one of two on Tuesday's ballot.
The other was Measure B, which asked the roughly 30,000 registered voters in the proposed incorporation zone whether they would prefer to elect their five city council members by district or at-large, provided the vote for cityhood is approved.
Sixteen candidates are vying for the seats.
Early returns showed voters were backing the at-large system.
City News Service contributed to this report.