Johnsen proposed to the Mad Love actress on a recent trip that the couple took to Napa, California, reports E Online.
"I'm really excited. I'm so happy," Greer said.
Greer met Johnsen, who works as co-executive producer of Real Time with Bill Maher, on a blind date in 2010.
No date has been set for the wedding.
Greer will soon be seen alongside Susan Sarandon and Jason Segel in the upcoming feature Jeff Who Lives At Home.
She currently voices the character of Cheryl Tunt on the FX animated series Archer.
Watch Judy Greer in the trailer for Henry's Crime below: