Johnson (1-0), a 24-year-old right-hander, allowed two runs on five hits over six innings, walking two en route to the victory in his second big-league appearance.
Johnson earned the triumph 23 years to the day after his father, former O's hurler Dave Johnson, notched his initial win for the team.
Mariners starter Kevin Millwood (4-10) was tagged for seven runs on eight hits in a four-inning effort.
Kyle Seager went 3-for-4 with a two-run shot for Seattle.
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