Moving to MGM in 1934, MacDonald reached a personal zenith in the title role of Lubitsch's saucy "The Merry Widow" (1934), but soon began playing more genteel if similarly princessy roles, in accordance with the stricter enforcement of Hollywood's self-imposed Production Code. She enjoyed considerable popular and critical acclaim with the delightful "Naughty Marietta" (1935), the first in a series of romantic operettas co-starring the handsome baritone Nelson Eddy. Mostly directed by W.S. Van Dyke, these films turned the couple into Hollywood's most popular singing team ever. Their best films together include the romantic valentine "Maytime" (1937) and the robust "Rose Marie" (1936), which featured the famous "Indian Love Call" so often parodied in later years. Later films, however, suffered from being overly schmaltzy or overproduced, and the team's popularity declined abruptly in the early 40s.
MacDonald also played in a number of enjoyable films sans Eddy while at MGM, including the lilting musical melodrama "The Firefly" (1937) with Allan Jones, and the memorable recreation of the Barbary Coast days of "San Francisco" (1936) just before the legendary 1906 earthquake. Famous for her powerful "high C", MacDonald turned to a successful concert hall career during the WWII years as her screen stardom waned but made occasional screen appearances through the late 40s. She married actor Gene Raymond in 1937 and the two later acted together onscreen in the third version of "Smilin' Through" in 1941. MacDonald's sister was character actress Marie Blake, who played the switchboard operator in the famous "Dr. Kildare" series of the 30s and 40s and later resurfaced under the name of Blossom Rock as the grandmother on the TV sitcom, "The Addams Family" (1964-66).