In Coburg on 23 December 1832, Marie became the second wife of 48-year-old Ernest I, Duke of Saxe-Coburg and Gotha. Ernest had been eager to find a new bride after the death of his first, estranged wife, Louise of Saxe-Gotha-Altenburg. While Ernest initially sought a wife of high status, he found that his age and poor reputation limited his choices. He settled for Marie, who was thirty-three years old and his niece – uncle-niece relationships were by this time becoming discouraged among European royalty, and Marie was the daughter of Ernest's sister Antoinette.
As a result of this union, Marie became stepmother of Ernest II and Prince Albert, future husband of Queen Victoria of the United Kingdom.[4][5] Marie was also their first-cousin. Ernest and his sons met Marie at Thalwitz Castle and accompanied her to the duchies to begin her marriage.[3] She would maintain a happy relationship until death with both of her stepsons,[2][6] becoming godmother (in absentia) of Victoria and Albert's first son, Albert Edward, Prince of Wales, (King Edward VII) in 1841.[7]
Historian Gillian Gill describes Marie as a "severe and melancholy lady". Marie and Ernest would have no children, and the two quickly grew apart, largely living on separate estates. By 1843 Marie had adopted a child of "humble parentage", though Albert cautioned her to avoid giving it hopeless aspirations to rank. He wrote, "I wish you more success than generally attends the education of poor children of the lower ranks by persons of our own."