Steele, 33, finished on 18 under par thanks to a seven-under 65, birdying the last three holes to beat overnight leader Patton Kizzire by a stroke.
Casey, 39, tied for third on 16 under with Americans Michael Kim, Johnson Wagner and Scott Piercy.
Scotland's Martin Laird and American Phil Mickelson finished on 14 under.
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England's Paul Casey claimed his fourth successive top-four finish without victory as American Brendan Steele won the Safeway Open in California.