While taking evening classes at Brooklyn College she met and married her teacher, James Meagher. He was an alcoholic and they had a difficult relationship: "It was a tragedy, as he was a most talented and admirable person in every way, and a poet, but could not resist the addiction and finally stopped trying."
Sylvia Meagher was a research analyst at the UNs World Health Organization. She took a strong interest in the assassination of John F. Kennedy and read the twenty-six volumes of the hearings and exhibits of the Warren Commission: "It was appalling to find how many of the Commission's statements were unsupportable or even completely contradicted by the testimony and/or exhibits... I began to list what is now a long list of deliberate misrepresentations, omissions, distortions, and other defects demonstrating not only extreme bias, incompetence, and carelessness but irrefutable instances of dishonesty."
In 1965 Meagher published Subject Index to the Warren Report and Hearings and Exhibits. As Meagher pointed out, studying the entire twenty-six volumes without a subject index would be "tantamount to a search for information in the Encylopedia Britannica if the contents were untitled, unalphabetized, and in random sequence."
A deep study of the Warren Commission Report convinced her that the its detailed evidence contradicted its general conclusions. Meagher therefore published Accessories After the Fact: The Warren Commission, the Authorities, and the Report (1967). Meagher was unconvinced that Lee Harvey Oswald had been a lone gunman and concluded that the Warren Commission had attempted to cover-up details of the real people behind the assassination. Meagher believed that John F. Kennedy had been killed by a group Anti-Castro exiles.
Meagher helped Mark Lane (Rush to Judgment), Léo Sauvage (The Oswald Affair - an Examination of the Contradictions and Omissions of the Warren Report) and Edward Jay Epstein (Inquest: The Warren Commission and the Establishment of Truth) in their research. In his book Sauvage commented: “I wish to express my gratitude to Mrs. Sylvia Meagher, author of an indispensable Subject Index and the only person in the world who really knows every item hidden in the twenty-six volumes of Hearings and Exhibits… With total unselfishness, Mrs. Meagher has always been available to me as to others, for any needed information, verification, or reference.”
In 1975 Richard Schweiker, who later became a member of the House Select Committee on Assassinations, pointed out that the Accessories After the Fact: The Warren Commission, the Authorities, and the Report "clearly establish Sylvia Meaghers major contribution to understanding this tragic incident in our nations history... and was instrumental in finally causing a committee of Congress - with full subpoena power, access to classified documents, and a working knowledge of the nuances of the FBI and CIA - to take a second official look at what happened in Dallas November 22, 1963.
In 1980 Meagher co-authored with Gary Owen, the Master Index to the John F.Kennedy Assassination Investigations. This book incorporated the House Select Committee on Assassinations volumes with the original Warren Commission Report. The lawyer, Russell Stetler, commented: "To the FBI agents in Dallas - who at least were doing their research on company time-the thought of plowing through thousands of pages of unindexed reference material was indeed daunting. Should we not pause to imagine how intimidating such work looked to spare-time researchers, that first generation of Warren Commission critics? Sylvia Meaghers index to the volumes not only enabled many researchers to get to work, pushed them over the first hurdle, so to speak; her efforts also provided a model of scholarly rigor and selfless personal dedication which has only grown more stunning with the passage of time."