She is possibly best known for her roles in film as Elsa Knudsen in Sam Peckinpah's Ride the High Country (1962), Susan Clabon in Alfred Hitchcock's Marnie (1964), and Betty Lloyd in John Sturges' Marooned (1969).

Serling answered the call himself, chose to visit and speak in her classroom, and years later remembering their previous interaction, cast Hartley in an episode ("The Long Morrow") of The Twilight Zone.

[2] Hartley's first credited film appearance was alongside Randolph Scott and Joel McCrea in the 1962 Sam Peckinpah Western Ride the High Country; the role earned her a BAFTA award nomination.

Hartley also guest-starred in numerous TV series during the decade, with appearances in Gunsmoke (five times including the title character in “Cotter’s Girl” in 1962); The Travels of Jaimie McPheeters; Death Valley Days; Judd, for the Defense; Bonanza; and Star Trek (as Zarabeth, Spock's love interest in S3 E23 "All Our Yesterdays", which aired on 3/13/1969) [9] among others.

Hartley continued to perform in film and TV during the 1970s, including two Westerns alongside Lee Van Cleef, Barquero (1970) and The Magnificent Seven Ride!

; McCloud; Little House on the Prairie; Love, American Style; Police Woman; and Columbo (1974’s Publish or Perish co-starring Jack Cassidy and 1977’s Try and Catch Me with Ruth Gordon).

In 1977, Hartley appeared in the TV movie The Last Hurrah, a political drama based on the Edwin O'Connor novel of the same name; and earned her first Emmy Award nomination.

She returned to the stage in 2014 as Eleanor of Aquitaine (with Ian Buchanan as Henry) in the Colony Theater Company production of James Goldman's The Lion in Winter.

In the late 1970s and early 1980s, Hartley appeared with James Garner in a popular series of television commercials advertising Polaroid cameras.

"[citation needed] Between 2001 and 2006, Hartley endorsed the See Clearly Method, a commercial eye exercise program, whose sales were halted by an Iowa court after a finding of fraudulent business practices and advertising.

[16] Hartley and Sroka co-wrote and starred in a romantic comedy based on their lives titled Our Almost Completely True Story, released in 2022.

[17] In her 1990 autobiography Breaking the Silence, written with Anne Commire, Hartley talked about her struggles with psychological problems, pointing directly to her grandfather's (Dr. Watson) practical application of his theories as the source of the dysfunction in his family.