She wrote her two earliest sf novels, The Space Guardian (1978) and Offworld (1979) under the name Max Daniels. Later, under her own name, she co-authored several fantasy novels with Mercedes Lackey.
The family obituary supplies more details about her achievements.
She worked both as a freelance scientific copy editor and as a research chemist for Foster D. Snell for many years, where she and her inventing partner developed aerosolized shaving foam and pink hair dye, among other creations, before she entered a second career as a best-selling author. Starting with Knight’s Honor in 1964, Roberta was the author of nearly fifty novels. These included more than twenty historical romances, including the Roselynde Chronicles and the Heiress Series, a number of medieval murder mysteries, and several historical fantasies. Other novels ranged from space opera to gothic romance to a mystery featuring the much-maligned Lucrezia Borgia as an amateur detective. She mostly wrote under her own name, but occasionally wrote as Max Daniels, Leah Jacobs, and Priscilla Hamilton. She won numerous awards for her writing, including a 1983 award from Romantic Times for Best Historical Series and a 1996 Lifetime Achievement Award from the Romance Writers of America.
Already well advanced in her writing career, Gellis’ first genre short fiction sale was to Marion Zimmer Bradley’s Fantasy Magazine and appeared in 1994. Her last sf/f story, the novelette “Renaissance Faire” (2005), was published in a collection co-edited by Andre Norton and Jean Rabe.