Downey, 49, chronicled Elsie Ann's struggle to find success as an actor, battles with addiction, and how she ultimately inspired him to get sober, said The Hollywood Reporter.
Downey praised the work she and his father, director Robert Downey Sr, did together, including 1975's 'Two Tons of Turquoise to Taos Tonight', which saw her play 17 characters. She often co-wrote with her then-husband and appeared in his films.
"By the mid '70s, the downside of drug culture caught up with many artists. She was an alcoholic. As the marriage suffered, she continued to work, but not for long. A recurring role on Mary 'Hartman, Mary Hartman' ('76-'77) was her last paying job... Not that she cared, she'd have done it for free," he wrote in a Facebook post.
Downey went on to talk about the final years of his mother's life, when she moved to Los Angeles to be with him and his family, but was plagued by health problems.
"Many fond memories of her in the last few years... Holidays, kid-stuff, her strutting around with a walking stick. I knew it was difficult, and understood as the visits got shorter."