Born in Portland, Oregon, she was a descendant of one of the first Spanish families of California. Her maternal great-grandfather was Mario Peralta, a founder of Oakland, California. Ynez was in motion pictures by the age of four, having a role as Little Kathyin The Miser's Heart (1911). She remained on the screen until the late 1940s, appearing in twenty-six movies. Her final role was an uncredited one in Samson and Delilah (1949).
Ynez' mother, Charlotte, appeared in stage productions in Los Angeles, California and in Hollywood films. Her father, Forest Seabury, was a character actor who once played in stock productions in the eastern United States. The comic actor Berton Churchill acted with Forest and later with Ynez when she made Allas the Deacon (1927), a comedy production of the Hollywood Playhouse. He once told Ynez about holding her as a baby of a year and a half. They watched her father act while standing in the wings. Miss Seabury began her stage career when she was but two years of age.
She trained under such masters as D.W. Griffith. Ynez participated in His Blossom Bride, a romantic drama of the stage. It was produced by Richard Walton Tully and premiered at the Mason opera house in Los Angeles in March 1928. The scenery and lighting for the play showed an opening prologue in the Painted Desert of Arizona and the Hopi Indian reservation. Members of the Hopi tribe were adopted by Ynez, who portrayed the Indian heroine. She was revered by the Hopi because of her understanding of their lives and ambitions. Before serving as background actors in the production, twenty-nine tribesmen and their chief toured Los Angeles in Cadillacs and La Salles.
The theme of American Indians was also central to Red Clay (1924), a film which starred William Desmond and Albert J. Smith. The plot was constructed around an Indian's education and his subsequent social ostracism. In her role as the Indian maid Miss Seabury earned acclaim for the very fine emotional quality of her work.
On November 3, 1928 Ynez wed broker Walter William Costello. The marriage culminated a romance of a year.