The actor opened up about her health struggles earlier this month when she posted about her hair loss.
However, her Instagram post led fans to believe she was still living with the disease - something she has now cleared up.
Writing on Instagram, Marcia revealed she had successfully completed treatment.
“I am so sorry my post wasn’t clear. I am POST cancer. All good now,” she wrote. “Hard journey, but I am HEALTHY, happy and more present and grateful than ever.
“Thank you from the bottom of my ever expanding heart for all of your LOVE.”
She continued: “After posting a picture of myself here and uttering the words #cancer and #hairloss, I feel liberated, deliriously free and completely me.
“How or why this simple act gave me such a powerful life I did not question, but I thanked God or whatever gave me the internal push to move forward and expose myself.
“I certainly wasn’t expecting the response of love and kindness that flowed to me. My capacity to receive requires a massive expansion! I am ecstatically alive and what interests me post cancer is #AUTHENTICITY, #VULNERABILITY, #TRANSPARENCY and of course #LOVE.”
She added: “Thank you again for your love and for sharing your personal stories of struggle with #cancer and #hairloss and getting your footing in the world.”
Marcia is best known for playing Bree Van de Kamp on ‘Desperate Housewives’ from 2004 to 2012, but has more recently appeared on ‘Quantico’ and ’Young & Consequences’.
Her husband, Tom Mahoney, previously overcame cancer in 2010.