According to the AP recap of Seattle's 2-0 win over the Angels, the Mariners tied a dubious mark also held by the 1969 Expos.
The Mariners had dropped 20 straight games to AL West opponents. The last win over an AL West team had been back on June 11, also against the Angels.
The Seattle lost three straight to Oakland (3). Then they went 8-4 against NL West teams. Then they lost three to Oakland (6). Then an 11-10 stretch against the AL East and Cleveland. Then three more losses to Oakland (9). Then a sweep of Tampa Bay. Then 11 straight losses to Texas, Oakland, and the Angels (20). The Mariners won 5 of 6 against the Yankees and Red Sox before their lightning fast win (1:51) against the Angels.
Monday's win was the quickest game at Safeco Field by one minute. The previous record was set on June 30 and I was there!.
When the Expos lost 20 straight games to the NL West, they did it in a much simpler way. They played all the games in a row. After beating the Reds on May 10, 1969, the Expos lost 20 in a row, two against Houston, two against Atlanta, three at Houston, three at Cincinnati, three versus Los Angeles, three versus San Diego, two versus San Francisco, and two at Los Angeles, before snapping the string at Dodger Stadium on June 8, 1969, 4-3. Jerry Robertson got the win and Elroy Face picked up the save. The Expos then won their next game against the NL West as well, winning at San Diego 7-4.
The 1969 Expos went 16-56 against the NL West en route to a 52-110 record. They lost all 12 games they played in Cincinnati and Houston.