|Portrayed by||Crista Flanagan|
|First appearance||The Hobo Code|
|Final appearance||The Color Blue|
|Employer||Sterling Cooper Advertising Agency|
|Occupation||Switch Board Operator, Secretary|
Lois Sadler was a switch board operator and eventually a secretary at Sterling Cooper.
As switchboard operator, she often listened in on a phone conversation between Salvatore Romano and his mother as they speak Italian, and asked the other secretaries about him, obviously smitten with him. Later, she ran into Sal when visiting the art department and bids him farewell by saying "ciao, ciao", which she heard him say on the phone.
After Peggy became a copy writer, Lois became Don's secretary. She was later reprimanded by Peggy for making a snide remark after she asks where Don was. Later, Joan chastises her for crying at work because of what Peggy said, telling her to do it at home. She was eventually fired by Don for incompetence, or, as he put it, for not "managing people's expectations."
Later, when she was questioned by Ken, Paul, Sal, and Harry about certain unusual activities happening at Sterling Cooper, she informed them of the merger with Puttnam, Powell, and Lowe. She also made them promise to take her off of the switchboard if any of them managed to keep their jobs.
After the merger, she became Paul's secretary. She was enlisted by Paul, Harry, and Ken to prank call Peggy in response to Peggy's want ad for a roommate. Later, while celebrating at Joan's farewell party, she took a turn in driving the John Deere mower that Ken had brought in after landing an account with them. The inexperienced secretary has difficulty in controlling her steering and ended up running over the foot of Guy MacKendrick, a British man who was poised to replace Lane Pryce as head of Sterling Cooper. Miraculously, she was not fired after this, as she was seen rousing a drunken Paul.