The Cubs outrighted Simmons to Triple-A Iowa after he cleared waivers Monday.
Simmons signed a minor-league contract with the Cubs on Wednesday, Bob Nightengale of USA Today Sports reports.
The Mariners declined to tender Simmons a contract for 2018, Greg Johns of MLB.com reports.
Simmons fired a scoreless eighth inning in Wednesday's 8-6 loss to the Rangers, striking out the side.
Simmons (forearm) fired a scoreless ninth inning in Sunday's 10-2 win over the Athletics, allowing a hit and recording two strikeouts.
Simmons (forearm) was activated from the 60-day DL on Friday.
Simmons (forearm, shoulder) fired a scoreless seventh inning in Triple-A Tacoma's loss to Memphis on Friday, his first appearance for the Rainiers.
Simmons (forearm), who recently suffered a setback in his rehab, threw a 30-pitch bullpen session on Saturday, Greg Johns of MLB.com reports.
Simmons (forearm) suffered a setback in his rehab, Bob Dutton of The Tacoma News Tribune reports.
Simmons was scratched from his scheduled rehab start Monday with Triple-A Tacoma due to arm soreness, Ryan Divish of The Seattle Times reports.