The two veterans, who will likely platoon at first base, were invited to spring training as non-roster players.
The Brewers elected to go with the veteran pair over former Cincinnati Reds farmhand Juan Francisco, who also was competing for a job at first, despite his .346 average with three home runs and six RBIs this spring.
Overbay had three hits in the game against the Reds on Sunday but was hitting .114 coming into the game. Reynolds is hitting .250 with two home runs and 11 RBIs.
"He swung the bat very well in BP today," Milwaukee manager Ron Roenicke said. "I don't know if telling him took pressure off even with a veteran you get comfortable, but it is good to see him swing that bat."
The Associated Press contributed to this report.