Rob Johnson began his professional career as a fourth round draft pick of the Seattle Mariners in the 2004 draft. He attended Saddleback College in California before transferring to the University of Houston where he played one season before entering the MLB Draft in 2004.
In the 2005 season, between A and high A ball, he posted a .280/.332/.432 slash line. The organization noticed and he earned the jump to Triple-A Tacoma the next season. He may have been promoted before he was ready though as he struggled with a line of .231/.261/.318. Over the next two seasons with Tacoma, his Triple-A numbers improved, posting a line of .305/.363/.441 with 9 HR in 2008.