Heineman signed a minor-league deal with the Mariners on Thursday, Mike Curto of the Triple-A Tacoma broadcast team reports.
Heineman was released by the Reds on Monday and is expected to head to Japan.
Heineman cleared waivers and was outrighted to Triple-A Louisville on Thursday.
Heineman was designated for assignment by the Reds on Monday.
Heineman was recalled from Triple-A Cincinnati on Thursday.
The Reds optioned Heineman to Triple-A Louisville on Tuesday.
Heineman went 1-for-4 with a two-run home run in Friday's 11-5 win over Colorado.
Heineman went 1-for-3 with a solo home run in Saturday's 4-3 loss to the Brewers.
Heineman was recalled by the Reds on Friday.
Heineman was optioned to Triple-A Louisville on Sunday, C. Trent Rosecrans of The Athletic reports.