Quite a heavy hitter, the more I learn, the more fascinated I get.

On October 12, 1964, Mary Pinchot Meyer was murdered on the C&O;Canal towpath in Georgetown, age 43. Her friend Anne Truitt was in Tokyo but learned of the murder that night. Truitt immediately contacted James Angleton and told him Mary had a diary and where it was — following Mary’s instructions should anything happen to her. The diary was destroyed — although Bradlee claims he read it and it only contained sketches and ephemera because “she was some kind of artist.”

http://en.wikipedia.org/wiki/Mary_Pinchot_Meyer

More more depth:
http://mcadams.posc.mu.edu/death12.htm

“Two telephone calls that night from overseas added new dimensions to Mary’s death. The first came from President Kennedy’s press secretary, Pierre Salinger, in Paris. He expressed his particular sorrow and condolences, and it was only after that conversation was over that we realized that we hadn’t known that Pierre had been a friend of Mary’s. The second, from Anne Truitt, an artist/sculptor living in Tokyo, was completely understandable. She had been perhaps Mary’s closest friend, and after she and Tony had grieved together, she told us that Mary had asked her to take possession of a private diary ‘if anything ever happened to me.’ Anne asked if we had found any such diary, and we told her we hadn’t looked for anything, much less a diary. We didn’t start looking until the next morning, when Tony and I walked around the corner a few blocks to Mary’s house. It was locked, as we had expected, but when we got inside, we found Jim Angleton, and to our complete surprise he told us he, too, was looking for Mary’s diary” (267).

This has sparked the interests of many researchers of the Kennedy assassination, since James Angleton was a high ranking CIA official. Many question how James Angleton even knew of the existence of the diary, much less why he was there to retrieve it. It should be noted, though, that Angleton’s wife, Cicely Angleton was another close, personal friend of Mary Meyer, and it seems completely reasonable that Mary also asked Cicely to take possession of the Diary. Other Reports say that Mary’s ex-husband, Cord Meyer, was involved with the search (Nobilem and Rosenbaum 29). This is important because Cord Meyer was assistant deputy director of plans for the CIA. His department was known as the “dirty tricks department” (Nobilem and Rosenbaum 28).

And from James Truitt’s wiki:
http://en.wikipedia.org/wiki/James_Truitt

In 1962 Mary Pinchot Meyer, (Cord Meyer’s ex-wife) told Truitt that she was having an affair with President Kennedy. Truitt made notes of the conversation, which years later he showed to journalist Jay Gourley. The notes recorded an episode in July 1962 when Mary Pinchot and President John F. Kennedy smoked marijuana, and include mention of Mary forgetting her slip after one visit and having it mailed back to her in a White House envelope.

In early 1963, Truitt helped extricate Phillip Graham from an ill-advised appearance at a publishers’ conference in Arizona, where Graham mentioned Kennedy’s affair. {Deborah Davis, 3rd ed. 1991, p. 154} Graham committed suicide on August 3, 1963. Katharine Graham assumed ownership of the Washington Post.

Highly speculative but sane:
http://www.serendipity.li/cia/cia_lsd.html

There is something about the story of Mary Meyer-as-JFK’s LSD- mistress that, if true, is naggingly bothersome. Like the assassination itself, it demands clarification, insists on being solved. Recall the plans of Al Hubbard and Humphrey Osmond to make the world a better, more peaceful place with the application of psychedelic chemical therapy to certain hand- picked politicians, and their claims of some success. All this quite a few years before Mary Meyer’s similar campaign: was it a case of like thinking evolving from acid insight happening in two different places and times, or was Mary Meyer possibly acting as an agent for Hubbard and Osmond, or someone who was their direct agent?

Consider the Leary connection to Mary Meyer in light of his connections to Hubbard, Osmond, and Aldous Huxley. Consider Hubbard’s career as an undercover agent for various government agencies and defense related industries, including his connection to the CIA. Consider his later career spent fighting against the youth counter-culture that one would otherwise think he would have been proud of as being the fruit of his labors. Then consider Mary Meyer herself, estranged wife of one of the CIA’s seminal top operatives, and her affair with a president who developed a mutual distrust for the CIA, a president who swore he would shatter the CIA into a thousand pieces and scatter the remnants to the wind. A president believed by many to have been assassinated by that same CIA. Consider the CIA sex and drugs safehouse experiments conducted by George Hunter White. Consider the proposed use of LSD as a means of discrediting foreign leaders. Consider the list of substances developed by the CIA, or attempted to be developed by the CIA listed above.

Now consider this: was John F. Kennedy, president of the United States, the ultimate MK-ULTRA guinea pig? Was Mary Pinchot Meyer playing some sort of clandestine game, was she some sort of Mata Hari? Or was she perhaps unwittingly being used by someone in that capacity?

After all, if we can seriously consider a CIA ready, willing, and able to assassinate a president they came to see as a threat or a traitor, why not a CIA willing to dose a president with LSD and study him as a test subject? What other world leader could they try their theories out on while observing him in closely monitored, intimate situations?

Best site I found so far:
http://www.spartacus.schoolnet.co.uk/JFKmeyerM.htm

Allen W. Dulles made contact with Cord Meyer in 1951. He accepted the invitation to join the CIA. Dulles told Meyer he wanted him to work on a project that was so secret that he could not be told about it until he officially joined the organization. Meyer was to work under Frank Wisner, director of the Office of Policy Coordination (OPC). This became the espionage and counter-intelligence branch of the CIA. Wisner was told to create an organization that concentrated on “propaganda, economic warfare; preventive direct action, including sabotage, anti-sabotage, demolition and evacuation measures; subversion against hostile states, including assistance to underground resistance groups, and support of indigenous anti-Communist elements in threatened countries of the free world.”

Meyer became part of what became known as Operation Mockingbird, a CIA program to influence the mass media. According to Deborah Davis (Katharine the Great: Katharine Graham and the Washington Post): Meyer was Mockingbird’s “principal operative”.

Further:

In January, 1963, Philip Graham, the publisher of the Washington Post, attended a convention of American newspaper editors in Phoenix. Graham, who was suffering from alcoholism, disclosed at the meeting that John F. Kennedy was having an affair with Mary Meyer. No newspaper reported this incident but Kennedy decided to bring an end to the affair. However, they continued to see each other at social functions.

According to his biography, Flashbacks, Timothy Leary claims that Mary phoned him the day after Kennedy was assassinated: “They couldn’t control him any more. He was changing too fast. He was learning too much… They’ll cover everything up. I gotta come see you. I’m scared. I’m afraid.”

In the summer of 1964 Meyer told friends that she believed someone had been inside her house while she was away. On another occasion she told Elizabeth Eisenstein that “she thought she had seen somebody leaving as she walked in”. Mary reported these incidents to the police. Eisenstein said Mary was clearly frightened by these incidents.

On 12th October, 1964, Mary Pinchot Meyer was shot dead as she walked along the Chesapeake and Ohio towpath in Georgetown. Henry Wiggins, a car mechanic, was working on a vehicle on Canal Road, when he heard a woman shout out: “Someone help me, someone help me”. He then heard two gunshots. Wiggins ran to the edge of the wall overlooking the tow path. He later told police he saw “a black man in a light jacket, dark slacks, and a dark cap standing over the body of a white woman.”

Mary appeared to be killed by a professional hitman. The first bullet was fired at the back of the head. She did not die straight away. A second shot was fired into the heart. The evidence suggests that in both cases, the gun was virtually touching Mary’s body when it was fired. As the FBI expert testified, the “dark haloes on the skin around both entry wounds suggested they had been fired at close-range, possibly point-blank”.

From “An Interview With Deborah Davis”:
http://www.umsl.edu/~thomaskp/hpage.htm

Q: Tim Leary suggested that your book was originally censored and
pulped because of references to Mary Pinchot Meyer. Do you feel
that way and can you tell us a little bit about Mary Meyer?

A: Mary Pinchot Meyer was the sister of Ben Bradlee’s second
wife, Tony Pinchot. She was a very beautiful, talented artist who
was living in Washington in the early `65 and she was the lover,
I would say the principal lover, of John Kennedy, who was
President of the United States. He was supposedly very much in
love with her and wanted to divorce Jackie and marry her.
The Timothy Leary connection is interesting because at that
time in the early sixties there was a group of society-type women
in Washington who thought that if they could get men in power
involved in mind-altering drugs they could see the world in a
different way and this would end the Cold War and end all
warfare. It was a very ambitious plan and a lot of them got their
marijuana and LSD from Timothy Leary, who at that time was a
professor of psychology at Harvard and had access to these drugs.
At that time were very experimental and they were going around in
a lot of the elite circles. It didn’t have the same connotation
that it has today of the hard stuff, of the cocaine and the
heroin. This was all very beautiful and mind-expanding type
stuff. So she was involved with Kennedy and they used to
supposedly smoke marijuana together in Kennedy’s bedroom and I
think Leary said that she also gave him LSD, although I couldn’t
swear to that. 
Anyway, she got murdered. She was murdered a year after
Kennedy died. Kennedy was killed in November `63 and Mary Meyer
was killed in 1964. She was walking her dog in Georgetown through
a wooded area and she was stabbed to death. And they never found
the killer. Some young black man was put in jail for ten months,
held over until his trial and then he was acquitted because there
was no evidence. And they’ve never found the killer but people
who have investigated the case say that it had all the earmarks
of a professional assassination.

Q: She was, of course, married to Cord Meyer, who was an
intelligence agent.

A: She had been. She was divorced from him at the time she was
with Kennedy. She had been with Cord Meyer in Europe when they
were living. He was involved in a lot of counter-espionage over
in Europe and she was supposedly a security risk because she
tended to fall in love and have affairs with handsome men. She
had an affair with one Italian count who was supposedly an
intelligence agent and this constituted a security risk. And I
suppose that their divorce was partly caused by the gulf between
them because he really just couldn’t talk about his work. Anyway,
she was a very talented painter and very charming and beautiful
and Kennedy was in love with her.

It’s kind of reassuring to know that Tim Boucher has already covered everything, actually:
http://www.timboucher.com/journal/2005/07/10/did-jfk-drop-acid/

She and the President of the United States smoke two joints of marijuana, reportedly prompting the leader of the free world to say, “This isn’t at all like cocaine. I’ll get you some of that.” Once he is suitably “loosened up” — Leary has emphasized the need to put subjects in a “benevolent state” before turning them on — Mary dispenses to Jack a dose of LSD. As it starts to “kick in,” he goes out and stands on Harry Truman’s balcony overlooking the rose-garden fountain, a soothing sight before him….

Mary also gets repeated mentions in Mark Reibling’s classic “Tinker, Tailor, Stoner, Spy: Was Timothy Leary a CIA Agent?”
http://www.markriebling.com/leary.html