Johnson, seeking his ninth career PGA Tour victory, birdied the final two holes to grab the lead.
Charlie Beljan, who finished second at the event last year, was among a group of nine players who finished their rounds at Riviera Country Club knotted for second place at 4-under 67.
J.B. Holmes also stood at minus-4 after completing 16 holes.
Pat Perez also stood at 3-under after 17 holes.
Fifteen players will wrap up their first rounds beginning at 10:30 a.m. EST.
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