Applegate, 38, and Lenoble, 41, dated for two years before becoming engaged on Valentine's Day. No date has been set for the wedding. Applegate was previously married to actor Johnathon Schaech from 2001-2005, but that marriage produced no children.
The "Samantha Who?" credited Lenoble with helping her recover after undergoing a double mastectomy. She underwent the procedure after learning she had an early form of breast cancer. Even though it was only present in one breast, Applegate took the aggressive step because her mother was a breast cancer survivor.
"I'm very grateful to Martyn for coming along at a time that he did because he's been my rock through all of this," Applegate told People last year. "He gave me something to really want to live for and something to smile about."