Whitfield died at age 39 in September 2011 after battling non-Hodgkin lymphoma on and off for over a year.
Lawless joined Hollywood in publicly mourning Whitfield at the time, praising her fellow Spartacus castmember as "a gentle man who never said a bad word".
She has now recorded a video message calling for pledges in his honour for tonight (September 7)'s Stand Up for Cancer telethon.
Lawless memorialised Whitfield as "a loving husband and father, terrific actor, and an inspiration to us all" and said his death caused her to become a greater advocate for cancer research.
"No-one is immune to cancer," she said. "It strikes unexpectedly, changing lives. We must stand united against this devastating disease."
Stand Up to Cancer is being produced by Joel Gallen and Gwyneth Paltrow to raise funds for research and care for the ill.
Justin Timberlake, Tom Hanks, Julia Roberts, Halle Berry, Olympians Gabby Douglas and Missy Franklin, Jessica Biel and Jeremy Renner will appear as celebrity presenters on the telecast.
Coldplay, Taylor Swift, Alicia Keys and Tim McGraw have all been lined up to perform.
Stand Up to Cancer telethon airs tonight at 8/7c on ABC, CBS, Fox, NBC and various cable channels.
Watch a promo for Stand Up to Cancer below: