The 53-year-old actor actually proposed Maggie, 35, months ago, only recently gifting her with a ring while spending time in Hawaii, reported Us magazine.
"No wedding date has been set yet," a source said.
The couple is yet to comment on their relationship. McDermott was previously married to actress Shiva Rose for 14 years until their split in 2009. The exes share daughters Colette, 18, and Charlotte, 9. This will be the first marriage for the 'Nikita' alum.