Federal authorities say Postal Service agents saw Terrence McLean open envelopes and remove the contents while sorting mail in June at the Columbus center and put mail in his sock and down his pants.
Agents say the 37-year-old McLean punched and bit them when they arrested him.
McLean, who resigned as part of the plea, pleaded guilty Thursday to multiple counts of mail theft and receiving stolen mail and two counts of assaulting a federal officer. He faces up to 20 years in prison.
Investigators recovered 71 first-class letters, 20 gift cards and $341 in cash when searching McLean.