Nehlen, whose Twitter account has now been suspended, posted a photo of Meghan and her husband-to-be, Prince Harry, replacing the actress' face via Photoshop with an image of Cheddar Man, the dark-skinned man believed to be the first modern Briton.
He captioned the image: “Honey does this tie make my face look pale?”
Following the racist tweet, Patrick, who worked alongside Meghan in Suits, called the politician "a sad and sick man."
"Oh @pnehlen - you’re a sad and sick man with no sense of shame or class. Get a life," he wrote.
"And don’t go anywhere near MM - she’s got more power, strength, honour and compassion in her fingernail than you’ll ever know in this lifetime. Way above your weight class."
Meghan and Patrick became good friends after playing love interests, Rachel Zane and Mike Ross, on the popular American legal drama for the past six years.
Following the news of Meghan and Harry's engagement in November, the actor wrote a heartwarming tribute on Twitter: "Playing Meghan’s television partner for the better part of a decade uniquely qualifies me to say this: Your Royal Highness, you are a lucky man and I know your long life together will be joyful, productive and hilarious. Meghan, so happy for you, friend. Much love."