With a time of 1 min 49.55 sec over two rounds, Myhrer finished ahead of two Austrians, Marcel Hirscher, .06sec behind, and Jens Byggmark at .61sec off the lead.
The top three had finished in the same order after the first round.
Myhrer, 29, last season's slalom World Cup winner, picked up his fifth success on the circuit on what proved to be a fast piste.
"The conditions were a little more difficult on the second run, but I just tried to ski my best," Myhrer said.
He continued his winning run of last season, notching up his third success in a row over the two campaigns and he commented: "I was only taking a break.
I rested up as I had a bit of a bad back last year, but it's all right now and I've trained well this summer.
"There are still a few more races to go before Schladming (the world championships in February next year), but obviously that's going to be the big target for this season."
Overall World Cup winner Hirscher is already off to a flying start, with two podium finishes in as many races, having taken third place in the giant slalom in Solden, Austria, a fortnight ago.
Hirscher said: "Achieving that in Levi is something special for me. It will be good to race on a different piste because I've had enough of this humidity and damp now."
Italy's Manfred Molgg, second in Solden, underlined his return to form with a fourth-place finish, narrowly ahead of fellow-Italian Patrick Thaler.