Smith, 32, was designated for assignment by the Rays on Sunday. He was 1 for 4 in only three games with the Rays this season. Smith has a .271 career batting average in six seasons with the Chicago White Sox, the Angels and the Rays.
Mathis, 38, was 0 for 9 with five strikeouts in three games with Atlanta.
The Braves also optioned left-hander Tucker Davidson to Triple-A Gwinnett and activated left-hander Grant Dayton from the 10-day injured list. Davidson allowed three runs in six innings against the New York Mets on Tuesday night in his 2021 debut
Dayton will be active for Wednesday night's game against the Mets after recovering from a thigh inflammation.
Smith broke into the majors with the White Sox, where he played from 2016-18.
The Associated Press contributed to this report.