There are many critics out there that will say that her career ended when her relationship with Clint Eastwood did, but I'd like to go on the record as saying that I believe her career ended when she made her first film with the action star.
Nothing against Clint Eastwood or his movies, it's just that Sandra Locke was miscast in most of those roles. She was an actress that should have been doing more personal dramas and just looked out of place in action films and comedies that appealed to the mainstream movie goer.
She was nominated for an Oscar for her very first film, The Heart is a Lonely Hunter from 1968. That was the time period when movies not only broke rules, but took the time to develop their characters as well. Sondra Locke was the perfect fit for those gritty dramas that came out of that time period. Instead she ended up in Hollywood blockbusters that made her look awkward and out of place.
Blame it on luck, her agent, herself, I don't know. Just don't blame it on Clint. Had she not worked with him, I'm sure I would still be writing about her in this blog, as only a handful of actresses from that time period are remembered today. I just believe that Sondra Locke would have had better movies and performances to write about.