The cast of "Whats Wrong with Secretary Kim" includes the following actors and their respective roles:
- Park Seo-joon as Lee Young-joon / Lee Sung-hyun
- Moon Woo-jin as 9-year-old Lee Sung-hyun
- Park Min-young as Kim Mi-so
- Lee Tae-hwan as Lee Sung-yeon
- Kang Ki-young as Park Yoo-sik
- Hwang Chan-sung as Go Gwi-nam
- Kim Byung-ok as Chairman Lee
- Kim Ye-won as Sul Ma-eum
- Kim Jung-woon as Park Joon-hwan
- Lee Yoo-joon as Jung Chi-in
- Pyo Ye-jin as Kim Ji-ah
- Kim Hye-ok as Mrs. Choi (Young-joons mother)
- Lee Se-young as staff who talked behind Kim Mi-sos back (Ep. 14)
- Kim Ka-yeon as staff who talked behind Kim Mi-sos back (Ep. 14)
- Lee Byung-joon as Jang Jeong-do (Ep. 16)
- Seo Hyo-rim as Choi Seo-jin (Yoo-siks ex-wife)
