The A's prospect allowed two hits over eight scoreless frames as the Sacramento River Cats held on for a 2-1 win over the Tacoma Rainiers.
He struck out three and walked one while inducing 16 ground-ball outs. The former first-round Draft pick threw 65 of 102 pitches for strikes and did not allow a runner past second base.
Mortensen retired the first 10 batters before giving up a one-out single in the fourth inning. Former top pick Dustin Ackley led off the eighth with a single for the Rainiers' only other hit off the right-hander.
The River Cats collected only four hits. Leadoff man Corey Wimberley singled twice, stole his Pacific Coast League-leading 37th base and scored a run. Second baseman Eric Sogard delivered an RBI triple in the third.
Sacramento's Michael Taylor went 0-for-4, snapping his 17-game hitting streak, but he drove in the Cats' second run with a groundout.
Sacramento survived a scare in the ninth as reliever Jon Hunton gave up a pair of hits and a run before Michael Benacka struck out Matt Tuiasosopo to end the game and record his second save.
Mortensen improved to 11-3 and lowered his ERA to 3.50. The 25-year-old shook off a rough outing at Salt Lake in his previous start last Sunday. He allowed five runs -- four earned -- on a season-high 12 hits over 4 1/3 for his first loss since May 18. He had won six games in eight starts between the defeats.
Tacoma starter Michael Pineda (3-1) suffered his first Triple-A loss, despite matching a season high with 11 strikeouts. He allowed one run on one hit over five frames and did not walk a batter.
The River Cats have won five straight and seven of nine on their 12-game road trip. Friday night's victory gave them a half-game lead over second-place Fresno in the Pacific South Division and the top record in the 16-team circuit.